conditional assignment in java

Java Tutorial

Control statements, java object class, java inheritance, java polymorphism, java abstraction, java encapsulation, java oops misc.

Conditional AND

The operator is applied between two Boolean expressions. It is denoted by the two AND operators (&&). It returns true if and only if both expressions are true, else returns false.

Conditional OR

The operator is applied between two Boolean expressions. It is denoted by the two OR operator (||). It returns true if any of the expression is true, else returns false.

Let's create a Java program and use the conditional operator.

ConditionalOperatorExample.java

Ternary Operator

The meaning of ternary is composed of three parts. The ternary operator (? :) consists of three operands. It is used to evaluate Boolean expressions. The operator decides which value will be assigned to the variable. It is the only conditional operator that accepts three operands. It can be used instead of the if-else statement. It makes the code much more easy, readable, and shorter.

Note: Every code using an if-else statement cannot be replaced with a ternary operator.

The above statement states that if the condition returns true, expression1 gets executed, else the expression2 gets executed and the final result stored in a variable.

Let's understand the ternary operator through the flowchart.

TernaryOperatorExample.java

Let's see another example that evaluates the largest of three numbers using the ternary operator.

LargestNumberExample.java

In the above program, we have taken three variables x, y, and z having the values 69, 89, and 79, respectively. The expression (x > y) ? (x > z ? x : z) : (y > z ? y : z) evaluates the largest number among three numbers and store the final result in the variable largestNumber. Let's understand the execution order of the expression.

First, it checks the expression (x > y) . If it returns true the expression (x > z ? x : z) gets executed, else the expression (y > z ? y : z) gets executed.

When the expression (x > z ? x : z) gets executed, it further checks the condition x > z . If the condition returns true the value of x is returned, else the value of z is returned.

When the expression (y > z ? y : z) gets executed it further checks the condition y > z . If the condition returns true the value of y is returned, else the value of z is returned.

Therefore, we get the largest of three numbers using the ternary operator.

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if Construct J8 Home   «   if Construct

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• switch Construct     >>

In this lesson we take our first look at the conditional statements available in Java. Conditional statements allow us to evaluate an expression and execute some code dependant upon the outcome. There are two conditional statements we can use with Java, the if Construct covered here and the switch Construct which we will cover in the next lesson. The choice of which to use really depends on the expression being evaluated. There is also the ternary ?: special operator which acts like an if Construct and which we will cover after looking in detail at the if Construct.

The if Construct  Top

Create a conditional expression using one of the relational operators available in Java to test one operand against another or test the result of a logical operation. We can execute one set of statements if the boolean result of the expression evaluates to true and another if the expression evaluates to false .

The relational and logical operators are discussed in more detail in the Operators lesson.

The following table shows the different forms of the if construct that can be used. We are using blocks of code to wrap statements which is optional, but good practice and will be used here.

We will go through the different forms of the if construct one at a time, using some code to make understanding of the above table easier.

Simple if Construct  Top

In its simplest form, the if Construct will execute the statement or block of code following the conditional expression when the expression evaluates to true , otherwise execution will continue after the statement or block of code. Lets look at some code to illustrate how this works:

Running the SimpleIf class after adding 24 to Program arguments pane of the run configuration, produces the following output:

If you have entered something when running the interpreter you should see 2 lines of output as in the screenshot above. We will talk about what args.length means in the Arrays lesson but for now it's enough to know that we are checking the args[] string array for input. The first if construct above doesn't use a block and therefore is limited to a single statement. The second if construct above uses a block and therefore can have multiple statements. We are just showing the first if construct for information purposes. We recommend always using a block when using any conditional as in the second example as it makes code easier to read, easier to maintain and is not limited to single statements.

if....else Construct  Top

In this form, the if Construct will execute the statement or block of code following the conditional expression when the expression evaluates to true , otherwise will execute the statement or block of code following the else when the expression evaluates to false . Lets look at some code to see this in action:

Running the IfElse class after adding 48 to Program arguments pane of the run configuration, produces the following output:

If you have run the program twice with and without a parameter you should see 4 lines of output similar to the screenshot above. The first if....else construct above doesn't use a block and therefore is limited to a single statement for the if and else . The second if....else construct above uses a block and therefore can have multiple statements for the if and else . We are just showing the first if...else construct for information purposes. We recommend always using a block when using any conditional as in the second example as it makes code easier to read, easier to maintain and is not limited to single statements. We will not be using the first simpler form in any more examples but you can now see how to code the construct this way.

Nested if Construct  Top

A nested if Construct is the target of another if or else statement. With nested if s the else statement always refers to the nearest if Construct that is within the same block of code as the else and is not already associated with an else . Lets look at some code:

Running the NestedIf class after adding 51 to Program arguments pane of the run configuration, produces the following output:

The message displayed depends which branch of the nested if construct we branch to. As you can see we can go in as many levels as we like but even with all that lovely indentation coding multiple nested if constructs can get complex and confusing although the example above is a bit contrived :).

Multiple if....else Construct  Top

The nested if Construct can get confusing and out of hand and generally in programming if something looks too complicated there is an easier way to do it.

Lets rewrite part of the nested if construct above using the multiple if....else if construct:

Running the MultipleIfElse class after adding 50 to Program arguments pane of the run configuration, produces the following output:

The message displayed depends which branch of the multiple if....else if construct we branch to. This example is easier to follow than the multiple nested if of the previous example and also illustrates how we can mix the various if constructs.

The Ternary ?: Operator  Top

The tenary (takes three operands) ?: operator can be used to replace an if....else construct of the following form:

Lets examine some code to see how the ?: operator works:

Running the TernaryOperator class produces the following output:

You should see 2 lines of output with the same values showing how we can replace this form of the if....else construct with the ternary ?: operator.

Related Quiz

Fundamentals Quiz 11 - if Construct Quiz

Lesson 12 Complete

In this lesson we investigated the if and ternary conditional statements available for use in Java.

What's Next?

In the next lesson we look at the switch conditional statement.

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Assignment, Arithmetic, and Unary Operators

The simple assignment operator.

One of the most common operators that you'll encounter is the simple assignment operator " = ". You saw this operator in the Bicycle class; it assigns the value on its right to the operand on its left:

This operator can also be used on objects to assign object references , as discussed in Creating Objects .

The Arithmetic Operators

The Java programming language provides operators that perform addition, subtraction, multiplication, and division. There's a good chance you'll recognize them by their counterparts in basic mathematics. The only symbol that might look new to you is " % ", which divides one operand by another and returns the remainder as its result.

The following program, ArithmeticDemo , tests the arithmetic operators.

This program prints the following:

You can also combine the arithmetic operators with the simple assignment operator to create compound assignments . For example, x+=1; and x=x+1; both increment the value of x by 1.

The + operator can also be used for concatenating (joining) two strings together, as shown in the following ConcatDemo program:

By the end of this program, the variable thirdString contains "This is a concatenated string.", which gets printed to standard output.

The Unary Operators

The unary operators require only one operand; they perform various operations such as incrementing/decrementing a value by one, negating an expression, or inverting the value of a boolean.

The following program, UnaryDemo , tests the unary operators:

The increment/decrement operators can be applied before (prefix) or after (postfix) the operand. The code result++; and ++result; will both end in result being incremented by one. The only difference is that the prefix version ( ++result ) evaluates to the incremented value, whereas the postfix version ( result++ ) evaluates to the original value. If you are just performing a simple increment/decrement, it doesn't really matter which version you choose. But if you use this operator in part of a larger expression, the one that you choose may make a significant difference.

The following program, PrePostDemo , illustrates the prefix/postfix unary increment operator:

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Java Tutorial

Java methods, java classes, java file handling, java how to, java reference, java examples, java if ... else, java conditions and if statements.

You already know that Java supports the usual logical conditions from mathematics:

• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b
• Equal to a == b
• Not Equal to: a != b

You can use these conditions to perform different actions for different decisions.

Java has the following conditional statements:

• Use if to specify a block of code to be executed, if a specified condition is true
• Use else to specify a block of code to be executed, if the same condition is false
• Use else if to specify a new condition to test, if the first condition is false
• Use switch to specify many alternative blocks of code to be executed

The if Statement

Use the if statement to specify a block of Java code to be executed if a condition is true .

Note that if is in lowercase letters. Uppercase letters (If or IF) will generate an error.

In the example below, we test two values to find out if 20 is greater than 18. If the condition is true , print some text:

Try it Yourself »

We can also test variables:

Example explained

In the example above we use two variables, x and y , to test whether x is greater than y (using the > operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".

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The else Statement

Use the else statement to specify a block of code to be executed if the condition is false .

In the example above, time (20) is greater than 18, so the condition is false . Because of this, we move on to the else condition and print to the screen "Good evening". If the time was less than 18, the program would print "Good day".

The else if Statement

Use the else if statement to specify a new condition if the first condition is false .

In the example above, time (22) is greater than 10, so the first condition is false . The next condition, in the else if statement, is also false , so we move on to the else condition since condition1 and condition2 is both false - and print to the screen "Good evening".

However, if the time was 14, our program would print "Good day."

Test Yourself With Exercises

Print "Hello World" if x is greater than y .

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Java Conditional Operator

Java programming tutorial index.

The Java Conditional Operator selects one of two expressions for evaluation, which is based on the value of the first operands. It is also called  ternary operator because it takes three arguments.

The conditional operator is used to handling simple situations in a line.

The above syntax means that if the value given in Expression1 is true, then Expression2 will be evaluated; otherwise, expression3 will be evaluated.

Program to Show Conditional Operator Works

In the above example, the condition given in expression1 is false because the value of a is not equal to the value of b.

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Ternary operator in java: an overview.

We already discussed the Types of Operators , Relational operators , Arithmetic operators , and  Logical operators . In this article, we'll explore the syntax, providing a comprehensive overview along with practical examples of Ternary Operator.

To further enhance your understanding and application of ternary operator's concepts, consider enrolling in the best Java Certification Course , to gain knowledge about effective utilization of unary operators for improved problem-solving and time management.

What is a Ternary Operator in Java?

In Java, the ternary operator, also known as the conditional operator, is a shorthand way of writing an if-else statement . It is used to make code more concise and readable by evaluating a Boolean expression and returning one of two values based on the result of the evaluation. The ternary operator has the following syntax:

• Condition: It denotes the condition specified in an if statement.
• Expression1: If the condition is met, this expression will be saved in the Variable .
• Expression2: If the condition is false, this expression will be saved in the Variable.
• It stores the result returned by either expression in a variable.

Read More - Top 50 Java Interview Questions

Flowchart of Ternary Operation

Examples of Ternary Operators in Java

Example 1: public class ternaryoperatorexample { public static void main (string[] args) { int x = 10 ; int y = 20 ; int result = (x > y) x : y; system.out.println( "the maximum of " + x + " and " + y + " is: " + result); } } run code >>, explanation.

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Use of Ternary Operator in Java?

The ternary operator in Java is a concise and powerful tool used for conditional expressions. Its primary purpose is to simplify the syntax of certain if-else statements, making code more compact and often improving readability. Here are some common use cases for the ternary operator in Java:

1.) Conditional Assignment

2.) inline printing, 3.) nested if-else statement., advantages of ternary operator.

• Code Conciseness: Ternary operators reduce the amount of code required for simple conditional assignments.
• Improved Readability: In certain cases, the ternary operator can make the code more readable, especially when dealing with short conditional expressions.
• Inline Usage: Ternary operators can be used inline, making code more compact and reducing the need for additional lines.

Q1. Can the ternary operator be nested?

Q2. are there any performance differences between the ternary operator and if-else statements, q3. can the ternary operator handle multiple conditions, about author.

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Operators constitute the basic building block of any programming language. Java too provides many types of operators which can be used according to the need to perform various calculations and functions, be it logical, arithmetic, relational, etc. They are classified based on the functionality they provide.

Types of Operators: 

• Arithmetic Operators
• Unary Operators
• Assignment Operator
• Relational Operators
• Logical Operators
• Ternary Operator
• Bitwise Operators
• Shift Operators

This article explains all that one needs to know regarding Assignment Operators. 

Assignment Operators

These operators are used to assign values to a variable. The left side operand of the assignment operator is a variable, and the right side operand of the assignment operator is a value. The value on the right side must be of the same data type of the operand on the left side. Otherwise, the compiler will raise an error. This means that the assignment operators have right to left associativity, i.e., the value given on the right-hand side of the operator is assigned to the variable on the left. Therefore, the right-hand side value must be declared before using it or should be a constant. The general format of the assignment operator is, 

Types of Assignment Operators in Java

The Assignment Operator is generally of two types. They are:

1. Simple Assignment Operator: The Simple Assignment Operator is used with the “=” sign where the left side consists of the operand and the right side consists of a value. The value of the right side must be of the same data type that has been defined on the left side.

2. Compound Assignment Operator: The Compound Operator is used where +,-,*, and / is used along with the = operator.

Let’s look at each of the assignment operators and how they operate: 

1. (=) operator: 

This is the most straightforward assignment operator, which is used to assign the value on the right to the variable on the left. This is the basic definition of an assignment operator and how it functions. 

Syntax:  

Example:  

2. (+=) operator: 

This operator is a compound of ‘+’ and ‘=’ operators. It operates by adding the current value of the variable on the left to the value on the right and then assigning the result to the operand on the left. 

Note: The compound assignment operator in Java performs implicit type casting. Let’s consider a scenario where x is an int variable with a value of 5. int x = 5; If you want to add the double value 4.5 to the integer variable x and print its value, there are two methods to achieve this: Method 1: x = x + 4.5 Method 2: x += 4.5 As per the previous example, you might think both of them are equal. But in reality, Method 1 will throw a runtime error stating the “i ncompatible types: possible lossy conversion from double to int “, Method 2 will run without any error and prints 9 as output.

Reason for the Above Calculation

Method 1 will result in a runtime error stating “incompatible types: possible lossy conversion from double to int.” The reason is that the addition of an int and a double results in a double value. Assigning this double value back to the int variable x requires an explicit type casting because it may result in a loss of precision. Without the explicit cast, the compiler throws an error. Method 2 will run without any error and print the value 9 as output. The compound assignment operator += performs an implicit type conversion, also known as an automatic narrowing primitive conversion from double to int . It is equivalent to x = (int) (x + 4.5) , where the result of the addition is explicitly cast to an int . The fractional part of the double value is truncated, and the resulting int value is assigned back to x . It is advisable to use Method 2 ( x += 4.5 ) to avoid runtime errors and to obtain the desired output.

Same automatic narrowing primitive conversion is applicable for other compound assignment operators as well, including -= , *= , /= , and %= .

3. (-=) operator: 

This operator is a compound of ‘-‘ and ‘=’ operators. It operates by subtracting the variable’s value on the right from the current value of the variable on the left and then assigning the result to the operand on the left. 

4. (*=) operator:

 This operator is a compound of ‘*’ and ‘=’ operators. It operates by multiplying the current value of the variable on the left to the value on the right and then assigning the result to the operand on the left. 

5. (/=) operator: 

This operator is a compound of ‘/’ and ‘=’ operators. It operates by dividing the current value of the variable on the left by the value on the right and then assigning the quotient to the operand on the left. 

6. (%=) operator: 

This operator is a compound of ‘%’ and ‘=’ operators. It operates by dividing the current value of the variable on the left by the value on the right and then assigning the remainder to the operand on the left. 

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Destructuring assignment

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

Description

The object and array literal expressions provide an easy way to create ad hoc packages of data.

The destructuring assignment uses similar syntax but uses it on the left-hand side of the assignment instead. It defines which values to unpack from the sourced variable.

Similarly, you can destructure objects on the left-hand side of the assignment.

This capability is similar to features present in languages such as Perl and Python.

For features specific to array or object destructuring, refer to the individual examples below.

Binding and assignment

For both object and array destructuring, there are two kinds of destructuring patterns: binding pattern and assignment pattern , with slightly different syntaxes.

In binding patterns, the pattern starts with a declaration keyword ( var , let , or const ). Then, each individual property must either be bound to a variable or further destructured.

All variables share the same declaration, so if you want some variables to be re-assignable but others to be read-only, you may have to destructure twice — once with let , once with const .

In many other syntaxes where the language binds a variable for you, you can use a binding destructuring pattern. These include:

• The looping variable of for...in for...of , and for await...of loops;
• Function parameters;
• The catch binding variable.

In assignment patterns, the pattern does not start with a keyword. Each destructured property is assigned to a target of assignment — which may either be declared beforehand with var or let , or is a property of another object — in general, anything that can appear on the left-hand side of an assignment expression.

Note: The parentheses ( ... ) around the assignment statement are required when using object literal destructuring assignment without a declaration.

{ a, b } = { a: 1, b: 2 } is not valid stand-alone syntax, as the { a, b } on the left-hand side is considered a block and not an object literal according to the rules of expression statements . However, ({ a, b } = { a: 1, b: 2 }) is valid, as is const { a, b } = { a: 1, b: 2 } .

If your coding style does not include trailing semicolons, the ( ... ) expression needs to be preceded by a semicolon, or it may be used to execute a function on the previous line.

Note that the equivalent binding pattern of the code above is not valid syntax:

You can only use assignment patterns as the left-hand side of the assignment operator. You cannot use them with compound assignment operators such as += or *= .

Default value

Each destructured property can have a default value . The default value is used when the property is not present, or has value undefined . It is not used if the property has value null .

The default value can be any expression. It will only be evaluated when necessary.

Rest property

You can end a destructuring pattern with a rest property ...rest . This pattern will store all remaining properties of the object or array into a new object or array.

The rest property must be the last in the pattern, and must not have a trailing comma.

Array destructuring

Basic variable assignment, destructuring with more elements than the source.

In an array destructuring from an array of length N specified on the right-hand side of the assignment, if the number of variables specified on the left-hand side of the assignment is greater than N , only the first N variables are assigned values. The values of the remaining variables will be undefined.

Swapping variables

Two variables values can be swapped in one destructuring expression.

Without destructuring assignment, swapping two values requires a temporary variable (or, in some low-level languages, the XOR-swap trick ).

Parsing an array returned from a function

It's always been possible to return an array from a function. Destructuring can make working with an array return value more concise.

In this example, f() returns the values [1, 2] as its output, which can be parsed in a single line with destructuring.

Ignoring some returned values

You can ignore return values that you're not interested in:

You can also ignore all returned values:

Using a binding pattern as the rest property

The rest property of array destructuring assignment can be another array or object binding pattern. The inner destructuring destructures from the array created after collecting the rest elements, so you cannot access any properties present on the original iterable in this way.

These binding patterns can even be nested, as long as each rest property is the last in the list.

On the other hand, object destructuring can only have an identifier as the rest property.

Unpacking values from a regular expression match

When the regular expression exec() method finds a match, it returns an array containing first the entire matched portion of the string and then the portions of the string that matched each parenthesized group in the regular expression. Destructuring assignment allows you to unpack the parts out of this array easily, ignoring the full match if it is not needed.

Using array destructuring on any iterable

Array destructuring calls the iterable protocol of the right-hand side. Therefore, any iterable, not necessarily arrays, can be destructured.

Non-iterables cannot be destructured as arrays.

Iterables are only iterated until all bindings are assigned.

The rest binding is eagerly evaluated and creates a new array, instead of using the old iterable.

Object destructuring

Basic assignment, assigning to new variable names.

A property can be unpacked from an object and assigned to a variable with a different name than the object property.

Here, for example, const { p: foo } = o takes from the object o the property named p and assigns it to a local variable named foo .

Assigning to new variable names and providing default values

A property can be both

• Unpacked from an object and assigned to a variable with a different name.
• Assigned a default value in case the unpacked value is undefined .

Unpacking properties from objects passed as a function parameter

Objects passed into function parameters can also be unpacked into variables, which may then be accessed within the function body. As for object assignment, the destructuring syntax allows for the new variable to have the same name or a different name than the original property, and to assign default values for the case when the original object does not define the property.

Consider this object, which contains information about a user.

Here we show how to unpack a property of the passed object into a variable with the same name. The parameter value { id } indicates that the id property of the object passed to the function should be unpacked into a variable with the same name, which can then be used within the function.

You can define the name of the unpacked variable. Here we unpack the property named displayName , and rename it to dname for use within the function body.

Nested objects can also be unpacked. The example below shows the property fullname.firstName being unpacked into a variable called name .

Setting a function parameter's default value

Default values can be specified using = , and will be used as variable values if a specified property does not exist in the passed object.

Below we show a function where the default size is 'big' , default co-ordinates are x: 0, y: 0 and default radius is 25.

In the function signature for drawChart above, the destructured left-hand side has a default value of an empty object = {} .

You could have also written the function without that default. However, if you leave out that default value, the function will look for at least one argument to be supplied when invoked, whereas in its current form, you can call drawChart() without supplying any parameters. Otherwise, you need to at least supply an empty object literal.

For more information, see Default parameters > Destructured parameter with default value assignment .

Nested object and array destructuring

For of iteration and destructuring, computed object property names and destructuring.

Computed property names, like on object literals , can be used with destructuring.

Invalid JavaScript identifier as a property name

Destructuring can be used with property names that are not valid JavaScript identifiers by providing an alternative identifier that is valid.

Destructuring primitive values

Object destructuring is almost equivalent to property accessing . This means if you try to destruct a primitive value, the value will get wrapped into the corresponding wrapper object and the property is accessed on the wrapper object.

Same as accessing properties, destructuring null or undefined throws a TypeError .

This happens even when the pattern is empty.

Combined array and object destructuring

Array and object destructuring can be combined. Say you want the third element in the array props below, and then you want the name property in the object, you can do the following:

The prototype chain is looked up when the object is deconstructed

When deconstructing an object, if a property is not accessed in itself, it will continue to look up along the prototype chain.

Specifications

Browser compatibility.

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

• Assignment operators
• ES6 in Depth: Destructuring on hacks.mozilla.org (2015)

Announcing TypeScript 5.4

Daniel Rosenwasser

March 6th, 2024 0 1

Today we’re excited to announce the release of TypeScript 5.4!

If you’re not familiar with TypeScript, it’s a language that builds on top of JavaScript by making it possible to declare and describe types. Writing types in our code allows us to explain intent and have other tools check our code to catch mistakes like typos, issues with null and undefined , and more. Types also power TypeScript’s editor tooling like the auto-completion, code navigation, and refactorings that you might see in Visual Studio and VS Code. In fact, if you’ve been writing JavaScript in either of those editors, you’ve been using TypeScript all this time!

To get started using TypeScript through NuGet or through npm with the following command:

Here’s a quick list of what’s new in TypeScript 5.4!

Preserved Narrowing in Closures Following Last Assignments

The noinfer utility type, object.groupby and map.groupby, support for require() calls in --moduleresolution bundler and --module preserve, checked import attributes and assertions, quick fix for adding missing parameters, auto-import support for subpath imports.

• Upcoming 5.5 Deprecations

Notable Behavioral Changes

What’s new since the beta and rc.

Since the beta, we’ve updated the release notes to document new notable behavioral changes , including restrictions around enum compatibility, restrictions on enum member naming, and improvements in mapped type behavior.

Since the release candidate, we’ve documented our new auto-import support for subpath imports .

TypeScript can usually figure out a more specific type for a variable based on checks that you might perform. This process is called narrowing.

One common pain-point was that these narrowed types weren’t always preserved within function closures.

Here, TypeScript decided that it wasn’t "safe" to assume that url was actually a URL object in our callback function because it was mutated elsewhere; however, in this instance, that arrow function is always created after that assignment to url , and it’s also the last assignment to url .

TypeScript 5.4 takes advantage of this to make narrowing a little smarter. When parameters and let variables are used in non- hoisted functions, the type-checker will look for a last assignment point. If one is found, TypeScript can safely narrow from outside the containing function. What that means is the above example just works now.

Note that narrowing analysis doesn’t kick in if the variable is assigned anywhere in a nested function. This is because there’s no way to know for sure whether the function will be called later.

This should make lots of typical JavaScript code easier to express. You can read more about the change on GitHub .

When calling generic functions, TypeScript is able to infer type arguments from whatever you pass in.

One challenge, however, is that it is not always clear what the "best" type is to infer. This might lead to TypeScript rejecting valid calls, accepting questionable calls, or just reporting worse error messages when it catches a bug.

For example, let’s imagine a createStreetLight function that takes a list of color names, along with an optional default color.

What happens when we pass in a defaultColor that wasn’t in the original colors array? In this function, colors is supposed to be the "source of truth" and describe what can be passed to defaultColor .

In this call, type inference decided that "blue" was just as valid of a type as "red" or "yellow" or "green" . So instead of rejecting the call, TypeScript infers the type of C as "red" | "yellow" | "green" | "blue" . You might say that inference just blue up in our faces!

One way people currently deal with this is to add a separate type parameter that’s bounded by the existing type parameter.

This works, but is a little bit awkward because D probably won’t be used anywhere else in the signature for createStreetLight . While not bad in this case , using a type parameter only once in a signature is often a code smell.

That’s why TypeScript 5.4 introduces a new NoInfer<T> utility type. Surrounding a type in NoInfer<...> gives a signal to TypeScript not to dig in and match against the inner types to find candidates for type inference.

Using NoInfer , we can rewrite createStreetLight as something like this:

Excluding the type of defaultColor from being explored for inference means that "blue" never ends up as an inference candidate, and the type-checker can reject it.

You can see the specific changes in the implementing pull request , along with the initial implementation provided thanks to Mateusz Burzyński !

TypeScript 5.4 adds declarations for JavaScript’s new Object.groupBy and Map.groupBy static methods.

Object.groupBy takes an iterable, and a function that decides which "group" each element should be placed in. The function needs to make a "key" for each distinct group, and Object.groupBy uses that key to make an object where every key maps to an array with the original element in it.

So the following JavaScript:

is basically equivalent to writing this:

Map.groupBy is similar, but produces a Map instead of a plain object. This might be more desirable if you need the guarantees of Map s, you’re dealing with APIs that expect Map s, or you need to use any kind of key for grouping – not just keys that can be used as property names in JavaScript.

and just as before, you could have created myObj in an equivalent way:

Note that in the above example of Object.groupBy , the object produced uses all optional properties.

This is because there’s no way to guarantee in a general way that all the keys were produced by groupBy .

Note also that these methods are only accessible by configuring your target to esnext or adjusting your lib settings. We expect they will eventually be available under a stable es2024 target.

We’d like to extend a thanks to Kevin Gibbons for adding the declarations to these groupBy methods .

TypeScript has a moduleResolution option called bundler that is meant to model the way modern bundlers figure out which file an import path refers to. One of the limitations of the option is that it had to be paired with --module esnext , making it impossible to use the import ... = require(...) syntax.

That might not seem like a big deal if you’re planning on just writing standard ECMAScript import s, but there’s a difference when using a package with conditional exports .

In TypeScript 5.4, require() can now be used when setting the module setting to a new option called preserve .

Between --module preserve and --moduleResolution bundler , the two more accurately model what bundlers and runtimes like Bun will allow, and how they’ll perform module lookups. In fact, when using --module preserve , the bundler option will be implicitly set for --moduleResolution (along with --esModuleInterop and --resolveJsonModule )

Under --module preserve , an ECMAScript import will always be emitted as-is, and import ... = require(...) will be emitted as a require() call (though in practice you may not even use TypeScript for emit, since it’s likely you’ll be using a bundler for your code). This holds true regardless of the file extension of the containing file. So the output of this code:

should look something like this:

What this also means is that the syntax you choose directs how conditional exports are matched. So in the above example, if the package.json of some-package looks like this:

TypeScript will resolve these paths to [...]/some-package/esm/foo-from-import.mjs and [...]/some-package/cjs/bar-from-require.cjs .

For more information, you can read up on these new settings here .

Import attributes and assertions are now checked against the global ImportAttributes type. This means that runtimes can now more accurately describe the import attributes

This change was provided thanks to Oleksandr Tarasiuk .

TypeScript now has a quick fix to add a new parameter to functions that are called with too many arguments.

This can be useful when threading a new argument through several existing functions, which can be cumbersome today.

This quick fix was provided courtsey of Oleksandr Tarasiuk .

In Node.js, package.json supports a feature called "subpath imports" via a field called imports s . It’s a way to re-mapping paths inside of a package to other module paths. Conceptually, this is pretty similar to path-mapping, a featue that certain module bundlers and loaders support (and which TypeScript supports via a feature called paths ). The only difference is that subpath imports always have to start with a # .

TypeScript’s auto-imports feature previously did not consider paths in imports which could be frustrating. Instead, users might have to manually define paths in their tsconfig.json . However, thanks to a contribution from Emma Hamilton , TypeScript’s auto-imports now support subpath imports !

Upcoming Changes from TypeScript 5.0 Deprecations

TypeScript 5.0 deprecated the following options and behaviors:

• target: ES3
• importsNotUsedAsValues
• noImplicitUseStrict
• noStrictGenericChecks
• keyofStringsOnly
• suppressExcessPropertyErrors
• suppressImplicitAnyIndexErrors
• preserveValueImports
• prepend in project references
• implicitly OS-specific newLine

To continue using them, developers using TypeScript 5.0 and other more recent versions have had to specify a new option called ignoreDeprecations with the value "5.0" .

However, TypScript 5.4 will be the last version in which these will continue to function as normal. By TypeScript 5.5 (likely June 2024), these will become hard errors, and code using them will need to be migrated away.

For more information, you can read up on this plan on GitHub , which contains suggestions in how to best adapt your codebase.

This section highlights a set of noteworthy changes that should be acknowledged and understood as part of any upgrade. Sometimes it will highlight deprecations, removals, and new restrictions. It can also contain bug fixes that are functionally improvements, but which can also affect an existing build by introducing new errors.

lib.d.ts Changes

Types generated for the DOM may have an impact on type-checking your codebase. For more information, see the DOM updates for TypeScript 5.4 .

More Accurate Conditional Type Constraints

The following code no longer allows the second variable declaration in the function foo .

Previously, when TypeScript checked the initializer for second , it needed to determine whether IsArray<U> was assignable to the unit type false . While IsArray<U> isn’t compatible any obvious way, TypeScript looks at the constraint of that type as well. In a conditional type like T extends Foo ? TrueBranch : FalseBranch , where T is generic, the type system would look at the constraint of T , substitute it in for T itself, and decide on either the true or false branch.

But this behavior was inaccurate because it was overly-eager. Even if the constraint of T isn’t assignable to Foo , that doesn’t mean that it won’t be instantiated with something that is. And so the more correct behavior is to produce a union type for the constraint of the conditional type in cases where it can’t be proven that T never or always extends Foo.

TypeScript 5.4 adopts this more accuratre behavior. What this means in practice is that you may begin to find that some conditional type instances are no longer compatible with their branches.

You can read about the specific changes here .

More Aggressive Reduction of Intersections Between Type Variables and Primitive Types

TypeScript now reduces intersections with type variables and primitives more aggressively, depending on how the type variable’s constraint overlaps with those primitives.

For more information, see the change here .

Improved Checking Against Template Strings with Interpolations

TypeScript now more accurately checks whether or not strings are assignable to the placeholder slots of a template string type.

This behavior is more desirable, but may cause breaks in code using constructs like conditional types, where these rule changes are easy to witness.

See this change for more details.

Errors When Type-Only Imports Conflict with Local Values

Previously, TypeScript would permit the following code under isolatedModules if the import to Something only referred to a type.

However, it’s not safe for a single-file compilers to assume whether it’s "safe" to drop the import , even if the code is guaranteed to fail at runtime. In TypeScript 5.4, this code will trigger an error like the following:

The fix should be to either make a local rename, or, as the error states, add the type modifier to the import:

See more information on the change itself .

New Enum Assignability Restrictions

When two enums have the same declared names and enum member names, they were previously always considered compatible; however, when the values were known, TypeScript would silently allow them to have differing values.

TypeScript 5.4 tightens this restriction by requiring the values to be identical when they are known.

Additionally, there are new restrictions for when one of the enum members does not have a statically-known value. In these cases, the other enum must at least be implicitly numeric (e.g. it has no statically resolved initializer), or it is explicitly numeric (meaning TypeScript could resolve the value to something numeric). Practically speaking, what this means is that string enum members are only ever compatible with other string enums of the same value.

For more information, see the pull request that introduced this change .

Name Restrictions on Enum Members

TypeScript no longer allows enum members to use the names Infinity , -Infinity , or NaN .

See more details here .

Better Mapped Type Preservation Over Tuples with any Rest Elements

Previously, applying a mapped type with any into a tuple would create an any element type. This is undesirable and is now fixed.

For more information, see the fix along with the follow-on discussion around behavioral changes and further tweaks .

Emit Changes

While not a breaking change per-se, developers may have implicitly taken dependencies on TypeScript’s JavaScript or declaration emit outputs. The following are notable changes.

• Preserve type parameter names more often when shadowed
• Move complex parameter lists of async function into downlevel generator body
• Do not remove binding alias in function declarations
• ImportAttributes should go through the same emit phases when in an ImportTypeNode

What’s Next?

In the coming months, we’ll be working on TypeScript 5.5, and you can see our iteration plan available on GitHub . Our target release dates are public so you, your team, and the broader TypeScript community can schedule accordingly. You can also try out nightly releases on npm or use the latest version of TypeScript and JavaScript in Visual Studio Code .

But until then, TypeScript 5.4 is still the latest and greatest stable version, and we hope that it makes coding a joy for you!

Happy Hacking!

– Daniel Rosenwasser and the TypeScript Team

Daniel Rosenwasser Senior Program Manager, TypeScript

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