difference between assignment and declaration

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Declare vs. Assign in JavaScript

What’s the difference between declaring a constant or variable and assigning it a value? Here’s everything you need to know.

You’re just getting started with JavaScript, and you want to learn the difference between declaring a constant or variable and assigning (or initializing) it.

To declare a constant or variable is to give it an identifier so that you can reference it. To assign a value to a variable is to store information in it.

This beginner’s guide will teach you how to declare constants and variables, how to assign them values, and more.

In JavaScript, you can create constants and variables—basically, objects that hold information—and give them values that you can use later in your code.

You create a constant or variable by declaring it with a const , let , or var statement.

Declare a Constant

The const statement is for constants:

Constants cannot be redeclared or reassigned.

Declare a Variable With Let

The let statement is for variables with a block scope:

Variables declared with the let statement cannot be redeclared but can be reassigned.

Declare a Variable With Var

And the var statement is for variable with a function scope or global scope:

Variables declared with the var statement can be redeclared and reassigned.

Once you’ve declared a constant or variable, you can assign it a value with the assignment operator (=) .

The first time you assign a value to a constant or variable is called “assignment” or “initialization.”

You can initialize a constant or variable at declaration:

You can also declare your constant or variable empty, then initialize it post-declaration:

The Difference Between Declaring and Assigning

To declare a constant or a variable is to create a data object and give a name, so that you can reference it later in your code. To assign a constant or variable, on the other hand, is to give it a value.

Another name for declaration, if oversimplified, could be “naming it.” And another name for assignment could be “storing information in it.”

To Redeclare

In JavaScript, certain types of data objects can be declared more than once, or “redeclared.”

Constants cannot be redeclared within the same scope:

Neither can variables declared with the let statement:

However, variables declared with the var statement can be redeclared:

To Reassign

In JavaScript, certain types of data objects can be assigned values more than once, or “reassigned.”

Constants cannot be reassigned within the same scope:

Variables declared with let or var , on the other hand, can be reassigned within the same scope:

Whether it is a good idea to redeclare and reassign in your code is the subject of heated debate in the JavaScript development community.

Summing It Up

Thank you for reading this far and I hope this tutorial helped.

You now know the difference between declaring a constant or variable and assigning it a value. You also know when you can—and cannot—redeclare and reassign constants or variables depending on the type of statement that you used.

If you have any questions, be sure to leave a reply below.

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Quick Tip: How to Declare Variables in JavaScript

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Difference between Declaration, Initialization and Assignment

Declaration types, accidental global creation, hoisting and the temporal dead zone, frequently asked questions (faqs) about javascript variable declaration.

When learning JavaScript one of the basics is to understand how to use variables. Variables are containers for values of all possible types, e.g. number, string or array (see data types ). Every variable gets a name that can later be used inside your application (e.g. to read its value).

In this quick tip you’ll learn how to use variables and the differences between the various declarations.

Before we start learning the various declarations, lets look at the lifecycle of a variable.

• Declaration : The variable is registered using a given name within the corresponding scope (explained below – e.g. inside a function).
• Initialization : When you declare a variable it is automatically initialized, which means memory is allocated for the variable by the JavaScript engine.
• Assignment : This is when a specific value is assigned to the variable.

Note : while var has been available in JavaScript since its initial releast, let and const are only available in ES6 (ES2015) and up. See this page for browser compatibility.

This declaration is probably the most popular, as there was no alternative until ECMAScript 6 . Variables declared with var are available in the scope of the enclosing function. If there is no enclosing function, they are available globally.

This will cause an error ReferenceError: hello is not defined , as the variable hello is only available within the function sayHello . But the following will work, as the variable will be declared globally – in the same scope console.log(hello) is located:

let is the descendant of var in modern JavaScript. Its scope is not only limited to the enclosing function, but also to its enclosing block statement. A block statement is everything inside { and } , (e.g. an if condition or loop). The benefit of let is it reduces the possibility of errors, as variables are only available within a smaller scope.

This will cause an error ReferenceError: hello is not defined as hello is only available inside the enclosing block – in this case the if condition. But the following will work:

Technically a constant isn’t a variable. The particularity of a constant is that you need to assign a value when declaring it and there is no way to reassign it. A const is limited to the scope of the enclosing block, like let .

Constants should be used whenever a value must not change during the applications running time, as you’ll be notified by an error when trying to overwrite them.

You can write all of above named declarations in the global context (i.e. outside of any function), but even within a function, if you forget to write var , let or const before an assignment, the variable will automatically be global.

The above will output Hello World to the console, as there is no declaration before the assignment hello = and therefore the variable is globally available.

Note: To avoid accidentally declaring global variables you can use strict mode .

Another difference between var and let / const relates to variable hoisting . A variable declaration will always internally be hoisted (moved) to the top of the current scope. This means the following:

is equivalent to:

An indication of this behavior is that both examples will log undefined to the console. If var hello; wouldn’t always be on the top it would throw a ReferenceError .

This behavior called hoisting applies to var and also to let / const . As mentioned above, accessing a var variable before its declaration will return undefined as this is the value JavaScript assigns when initializing it.

But accessing a let / const variable before its declaration will throw an error. This is due to the fact that they aren’t accessible before their declaration in the code. The period between entering the variable’s scope and reaching their declaration is called the Temporal Dead Zone – i.e. the period in which the variable isn’t accessible.

You can read more about hoisting in the article Demystifying JavaScript Variable Scope and Hoisting .

To reduce susceptibility to errors you should use const and let whenever possible. If you really need to use var then be sure to move declarations to the top of the scope, as this avoids unwanted behavior related to hoisting.

What is the difference between variable declaration and initialization in JavaScript?

In JavaScript, variable declaration and initialization are two distinct steps in the process of using variables. Declaration is the process of introducing a new variable to the program. It’s done using the var, let, or const keywords. For example, let x; Here, x is declared but not defined. It’s like telling the program, “Hey, I’m going to use a variable named x.” Initialization, on the other hand, is the process of assigning a value to the declared variable for the first time. For example, x = 5; Here, x is initialized with the value 5. It’s like telling the program, “The variable x I told you about earlier? It’s value is 5.”

Can I declare a variable without initializing it in JavaScript?

Yes, in JavaScript, you can declare a variable without initializing it. When you declare a variable without assigning a value to it, JavaScript automatically assigns it the value of undefined. For example, if you declare a variable like this: let x; and then try to log x to the console, you’ll get undefined because x has been declared but not initialized.

What happens if I use a variable without declaring it in JavaScript?

In JavaScript, if you use a variable without declaring it first, you’ll get a ReferenceError. This is because JavaScript needs to know about a variable before it can be used. If you try to use a variable that hasn’t been declared, JavaScript doesn’t know what you’re referring to and throws an error. For example, if you try to log x to the console without declaring x first, you’ll get a ReferenceError: x is not defined.

What is the difference between var, let, and const in JavaScript variable declaration?

In JavaScript, var, let, and const are all used to declare variables, but they have different behaviors. var is function-scoped, meaning a variable declared with var is available within the function it’s declared in. let and const are block-scoped, meaning they’re only available within the block they’re declared in. Additionally, const is used to declare constants, or variables that can’t be reassigned after they’re initialized.

Can I redeclare a variable in JavaScript?

In JavaScript, whether you can redeclare a variable depends on how you initially declared it. If you declared a variable with var, you can redeclare it. However, if you declared a variable with let or const, you can’t redeclare it within the same scope. Attempting to do so will result in a SyntaxError.

What is hoisting in JavaScript?

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase. This means that you can use variables and functions before they’re declared. However, only the declarations are hoisted, not initializations. If a variable is declared and initialized after using it, the variable will be undefined.

What is the scope of a variable in JavaScript?

The scope of a variable in JavaScript determines where that variable can be accessed from within your code. Variables declared with var have function scope, meaning they can be accessed anywhere within the function they’re declared in. Variables declared with let and const have block scope, meaning they can only be accessed within the block they’re declared in.

What is the difference between null and undefined in JavaScript?

In JavaScript, null and undefined are both special values that represent the absence of a value. However, they’re used in slightly different ways. undefined is the value assigned to a variable that has been declared but not initialized. null, on the other hand, is a value that represents no value or no object. It needs to be assigned to a variable explicitly.

Can I use special characters in variable names in JavaScript?

In JavaScript, variable names can include letters, digits, underscores, and dollar signs. They must begin with a letter, underscore, or dollar sign. Special characters like !, @, #, %, etc., are not allowed in variable names.

What is a global variable in JavaScript?

A global variable in JavaScript is a variable that’s declared outside of any function or block. Because it’s not tied to a function or block, a global variable can be accessed from anywhere in your code. However, global variables can lead to issues with naming conflicts and are generally best avoided when possible.

Julian is a passionate software developer currently focusing on frontend technologies and loves open source.

1.4 — Variable assignment and initialization

In the previous lesson ( 1.3 -- Introduction to objects and variables ), we covered how to define a variable that we can use to store values. In this lesson, we’ll explore how to actually put values into variables and use those values.

As a reminder, here’s a short snippet that first allocates a single integer variable named x , then allocates two more integer variables named y and z :

Variable assignment

After a variable has been defined, you can give it a value (in a separate statement) using the = operator . This process is called assignment , and the = operator is called the assignment operator .

By default, assignment copies the value on the right-hand side of the = operator to the variable on the left-hand side of the operator. This is called copy assignment .

Here’s an example where we use assignment twice:

This prints:

When we assign value 7 to variable width , the value 5 that was there previously is overwritten. Normal variables can only hold one value at a time.

One of the most common mistakes that new programmers make is to confuse the assignment operator ( = ) with the equality operator ( == ). Assignment ( = ) is used to assign a value to a variable. Equality ( == ) is used to test whether two operands are equal in value.

Initialization

One downside of assignment is that it requires at least two statements: one to define the variable, and another to assign the value.

These two steps can be combined. When an object is defined, you can optionally give it an initial value. The process of specifying an initial value for an object is called initialization , and the syntax used to initialize an object is called an initializer .

In the above initialization of variable width , { 5 } is the initializer, and 5 is the initial value.

Different forms of initialization

Initialization in C++ is surprisingly complex, so we’ll present a simplified view here.

There are 6 basic ways to initialize variables in C++:

You may see the above forms written with different spacing (e.g. int d{7}; ). Whether you use extra spaces for readability or not is a matter of personal preference.

Default initialization

When no initializer is provided (such as for variable a above), this is called default initialization . In most cases, default initialization performs no initialization, and leaves a variable with an indeterminate value.

We’ll discuss this case further in lesson ( 1.6 -- Uninitialized variables and undefined behavior ).

Copy initialization

When an initial value is provided after an equals sign, this is called copy initialization . This form of initialization was inherited from C.

Much like copy assignment, this copies the value on the right-hand side of the equals into the variable being created on the left-hand side. In the above snippet, variable width will be initialized with value 5 .

Copy initialization had fallen out of favor in modern C++ due to being less efficient than other forms of initialization for some complex types. However, C++17 remedied the bulk of these issues, and copy initialization is now finding new advocates. You will also find it used in older code (especially code ported from C), or by developers who simply think it looks more natural and is easier to read.

For advanced readers

Copy initialization is also used whenever values are implicitly copied or converted, such as when passing arguments to a function by value, returning from a function by value, or catching exceptions by value.

Direct initialization

When an initial value is provided inside parenthesis, this is called direct initialization .

Direct initialization was initially introduced to allow for more efficient initialization of complex objects (those with class types, which we’ll cover in a future chapter). Just like copy initialization, direct initialization had fallen out of favor in modern C++, largely due to being superseded by list initialization. However, we now know that list initialization has a few quirks of its own, and so direct initialization is once again finding use in certain cases.

Direct initialization is also used when values are explicitly cast to another type.

One of the reasons direct initialization had fallen out of favor is because it makes it hard to differentiate variables from functions. For example:

List initialization

The modern way to initialize objects in C++ is to use a form of initialization that makes use of curly braces. This is called list initialization (or uniform initialization or brace initialization ).

List initialization comes in three forms:

As an aside…

Prior to the introduction of list initialization, some types of initialization required using copy initialization, and other types of initialization required using direct initialization. List initialization was introduced to provide a more consistent initialization syntax (which is why it is sometimes called “uniform initialization”) that works in most cases.

Additionally, list initialization provides a way to initialize objects with a list of values (which is why it is called “list initialization”). We show an example of this in lesson 16.2 -- Introduction to std::vector and list constructors .

List initialization has an added benefit: “narrowing conversions” in list initialization are ill-formed. This means that if you try to brace initialize a variable using a value that the variable can not safely hold, the compiler is required to produce a diagnostic (usually an error). For example:

In the above snippet, we’re trying to assign a number (4.5) that has a fractional part (the .5 part) to an integer variable (which can only hold numbers without fractional parts).

Copy and direct initialization would simply drop the fractional part, resulting in the initialization of value 4 into variable width . Your compiler may optionally warn you about this, since losing data is rarely desired. However, with list initialization, your compiler is required to generate a diagnostic in such cases.

Conversions that can be done without potential data loss are allowed.

To summarize, list initialization is generally preferred over the other initialization forms because it works in most cases (and is therefore most consistent), it disallows narrowing conversions, and it supports initialization with lists of values (something we’ll cover in a future lesson). While you are learning, we recommend sticking with list initialization (or value initialization).

Best practice

Prefer direct list initialization (or value initialization) for initializing your variables.

Author’s note

Bjarne Stroustrup (creator of C++) and Herb Sutter (C++ expert) also recommend using list initialization to initialize your variables.

In modern C++, there are some cases where list initialization does not work as expected. We cover one such case in lesson 16.2 -- Introduction to std::vector and list constructors .

Because of such quirks, some experienced developers now advocate for using a mix of copy, direct, and list initialization, depending on the circumstance. Once you are familiar enough with the language to understand the nuances of each initialization type and the reasoning behind such recommendations, you can evaluate on your own whether you find these arguments persuasive.

Value initialization and zero initialization

When a variable is initialized using empty braces, value initialization takes place. In most cases, value initialization will initialize the variable to zero (or empty, if that’s more appropriate for a given type). In such cases where zeroing occurs, this is called zero initialization .

Q: When should I initialize with { 0 } vs {}?

Use an explicit initialization value if you’re actually using that value.

Use value initialization if the value is temporary and will be replaced.

Initialize your variables

Initialize your variables upon creation. You may eventually find cases where you want to ignore this advice for a specific reason (e.g. a performance critical section of code that uses a lot of variables), and that’s okay, as long the choice is made deliberately.

Related content

For more discussion on this topic, Bjarne Stroustrup (creator of C++) and Herb Sutter (C++ expert) make this recommendation themselves here .

We explore what happens if you try to use a variable that doesn’t have a well-defined value in lesson 1.6 -- Uninitialized variables and undefined behavior .

Initialize your variables upon creation.

Initializing multiple variables

In the last section, we noted that it is possible to define multiple variables of the same type in a single statement by separating the names with a comma:

We also noted that best practice is to avoid this syntax altogether. However, since you may encounter other code that uses this style, it’s still useful to talk a little bit more about it, if for no other reason than to reinforce some of the reasons you should be avoiding it.

You can initialize multiple variables defined on the same line:

Unfortunately, there’s a common pitfall here that can occur when the programmer mistakenly tries to initialize both variables by using one initialization statement:

In the top statement, variable “a” will be left uninitialized, and the compiler may or may not complain. If it doesn’t, this is a great way to have your program intermittently crash or produce sporadic results. We’ll talk more about what happens if you use uninitialized variables shortly.

The best way to remember that this is wrong is to consider the case of direct initialization or brace initialization:

Because the parenthesis or braces are typically placed right next to the variable name, this makes it seem a little more clear that the value 5 is only being used to initialize variable b and d , not a or c .

Unused initialized variables warnings

Modern compilers will typically generate warnings if a variable is initialized but not used (since this is rarely desirable). And if “treat warnings as errors” is enabled, these warnings will be promoted to errors and cause the compilation to fail.

Consider the following innocent looking program:

When compiling this with the g++ compiler, the following error is generated:

and the program fails to compile.

There are a few easy ways to fix this.

• If the variable really is unused, then the easiest option is to remove the defintion of x (or comment it out). After all, if it’s not used, then removing it won’t affect anything.
• Another option is to simply use the variable somewhere:

But this requires some effort to write code that uses it, and has the downside of potentially changing your program’s behavior.

The [[maybe_unused]] attribute C++17

In some cases, neither of the above options are desirable. Consider the case where we have a bunch of math/physics values that we use in many different programs:

If we use these a lot, we probably have these saved somewhere and copy/paste/import them all together.

However, in any program where we don’t use all of these values, the compiler will complain about each variable that isn’t actually used. While we could go through and remove/comment out the unused ones for each program, this takes time and energy. And later if we need one that we’ve previously removed, we’ll have to go back and re-add it.

To address such cases, C++17 introduced the [[maybe_unused]] attribute, which allows us to tell the compiler that we’re okay with a variable being unused. The compiler will not generate unused variable warnings for such variables.

The following program should generate no warnings/errors:

Additionally, the compiler will likely optimize these variables out of the program, so they have no performance impact.

In future lessons, we’ll often define variables we don’t use again, in order to demonstrate certain concepts. Making use of [[maybe_unused]] allows us to do so without compilation warnings/errors.

Question #1

What is the difference between initialization and assignment?

Show Solution

Initialization gives a variable an initial value at the point when it is created. Assignment gives a variable a value at some point after the variable is created.

Question #2

What form of initialization should you prefer when you want to initialize a variable with a specific value?

Direct list initialization (aka. direct brace initialization).

Question #3

What are default initialization and value initialization? What is the behavior of each? Which should you prefer?

Default initialization is when a variable initialization has no initializer (e.g. int x; ). In most cases, the variable is left with an indeterminate value.

Value initialization is when a variable initialization has an empty brace (e.g. int x{}; ). In most cases this will perform zero-initialization.

You should prefer value initialization to default initialization.

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Initialization, declaration and assignment terms in Java

A variable provides us with named storage that our programs can manipulate. Each variable in Java has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

You must declare all variables before they can be used. Following is the basic form of a variable declaration − data type variable [ = value][, variable [ = value] ...] ;

Here data type is one of Java's datatypes and variable is the name of the variable. To declare more than one variable of the specified type, you can use a comma-separated list.

Following are valid examples of variable declaration and initialization in Java −

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Interview guide, recommendations, understanding variables in python: declaration, assignment, and naming conventions.

Variables are essential elements in programming languages like Python. they serve as containers to store data values that can be manipulated or referenced in a program. Understanding how variables are declared, assigned values, and named according to conventions is crucial for writing clean and readable Python code.

Variable Declaration and Assignment in Python

In Python, variables are declared by simply assigning a value to them. Unlike some other programming languages, Python does not require explicit declaration of variables or specifying their data types. the variable's data type is inferred based on the value assigned to it. this process is known as variable assignment, which can be done using the equals sign "=" (also known as the assignment operator). Once the value is assigned to a variable, it is created, and we can start using it in other statements or expressions.

for instance, the following code snippet creates a variable named " stockPrice " and assigns the value 100 (an integer) to it.

the variable stockPrice is now created and holds the value 100 . You can use it in other expressions or statements as shown below.

Variables in Python can be reassigned to different values, allowing for dynamic changes in a program. A variable's value can be updated by simply assigning a new value to it. You can re-declare a variable by assigning a new value to it. for instance, we can change the value of the " stockPrice " variable to 150 as follows:

You can also assign the values to multiple variables simultaneously using the chaining assignment operation as shown below.

the simultaneous assignment operation in Python provides programmers with a concise and efficient way to assign values to multiple variables in a single line of code.

Naming Conventions for Variables

While naming variables in Python, it is essential to follow proper naming conventions for code clarity and maintainability. Here are some guidelines to follow when naming variables:

Start with a Letter or Underscore: Variable names must begin with a letter (a-z, A-Z) or an underscore (_) character. for example: "stockPrice" or "_stockPrice" are acceptable variable names.

Avoid Starting with a Number: Variable names cannot begin with a number. for example: "1stockPrice" or "1_stockPrice" are invalid variable names.

Use Alphanumeric Characters and Underscores: Variable names can only contain alphanumeric characters (A-Z, a-z, 0-9) and underscores (_). for example: "stockPrice", "stock_Price", "stockPrice_1", and "stockPrice_2" are all valid variable names.

Avoid Whitespace and Special Characters: Variable names should not contain whitespace or special characters such as +, -, etc. for example: "stock price" or "stock-price" are invalid variable names.

Case Sensitive: Variable names are case-sensitive. for example: "StockPrice", "stockPrice", and "Stockprice" are considered distinct variables.

Avoid Python Keywords: Avoid using Python keywords as variable names. for example: keywords such as "str", "is", and "for" cannot be used as variable names as they are reserved keywords in Python.

In addition to these guidelines, professional programmers follow certain conventions to enhance code readability (best practices). these practices include using a name that describes the purpose " stockPrice ", instead of using dummy or temporary names " temp ". It's also common practice to separate words in variable names with underscores " stock_price ", and start variable names with lowercase letters " stockPrice ".

Following these guidelines and practices can make your code more readable and maintainable. Remember that these are good coding practices recommended by professional programmers, which can be applied to any programming language.

Declaration & Assignment

Kyle Simpson

Check out a free preview of the full JavaScript: The Recent Parts course

The "Declaration & Assignment" Lesson is part of the full, JavaScript: The Recent Parts course featured in this preview video. Here's what you'd learn in this lesson:

Kyle defined the difference between declaration and assignment, and explains both in the context of destructuring.

Transcript from the "Declaration & Assignment" Lesson

[00:00:00] >> Kyle Simpson: Okay, this destructuring pattern that we've been talking about has on the left hand side been var declarations as we went along. But if we've had already declared those variables, if I had already said, >> Kyle Simpson: That those variables existed, then I could have done those assignments without any var declarations, right?

[00:00:22] As a matter of fact, what we're getting at here is that the assignments aren't inherently related to declarations, that's just the convenience that you can do assignment along with declaration. The same is true of destructuring, we could have defined these variables first ahead of time and then done a destructuring separate of a declaration.

[00:00:43] So we could have said var first, second, third, and fourth. And then we could have just done a destructuring without any var statement on it. Destructuring is actually about the assignments, it's not about the declaration. >> Kyle Simpson: Of course, if we're gonna assign them to variables that already exist, we could also assign them to entirely other locations.

[00:01:20] Anything that would be valid to assign it to, in spec speak that's called a valid left-hand side target. Any place that we could assign it to validly would be okay. So we could have an o object here, var o = object. And then I could say o.first and o.second and o.third and o.fourth, right?

[00:01:49] >> Kyle Simpson: I could do the exact same thing on this side. I could have had an o object and I could have said o.first and o.second and o.third and o.fourth. >> Kyle Simpson: Of course, it could have targeted not an object but another array if I wanted to. So I wouldn't have said o.first, I would have said something like o(3).

[00:02:20] If o was an array, then we could have said o[3] and o [10] and o[42] and o[100]. And the exact same thing would be true over here. We could have an array and then have these references. >> Kyle Simpson: I suppose this would be technically more equivalent. >> Kyle Simpson: So in other words, array destructuring is just the assignment part, not the declaration part.

[00:03:35] And anything that we could validly assign to, it's okay to show up in the imperative forum, it's also okay to show up in the declarative form. What would have happened if I had done something like var o[3]? >> Kyle Simpson: It's a little bit of a trick question, that would've been a syntax error.

[00:04:01] That's not valid to show up on the left-hand side of an equals, I mean, it is valid to show up in left-hand segment. It's not valid to show up in a declaration. That's not a valid identifier that you would declare. So if it's not a valid thing that you would declare, it's also not valid to do the declaration with your destructuring syntax.

[00:04:21] And you see there that my SyntaxHighlighter is telling me, something's wrong here, that's not syntactically valid. >> Speaker 2: Yeah, on line 11 on the right-hand side, that would just put if there were five or six other items into the gather syntax, that would just put an array with those items at index 100.

[00:04:50] >> Kyle Simpson: Yeah, it's not flattening get out into the array, it just adding that array at that position. >> Kyle Simpson: All right, I said that we could actually put the assignment on either side. So in the same way that I could say temp = data, and then break it down, if I wanted to have the break down.

[00:05:20] >> Kyle Simpson: So if I wanted to assign the o, >> Kyle Simpson: And I wanted to have temp, then I could say temp = this and that equals data. Now here's where people are gonna often get confused is because if this was a subset, like if I left those out, people might get confused and think that temp is not gonna point at the entire array.

[00:05:55] And I think that temp is only gonna point at the subset that we see in the pattern. But you have to understand that the way assignment expressions work is that an assignment expression, whatever it is, if it's x equals three, the result of the assignment expression is the entire value that was subject to assignment.

[00:06:17] So the result of this assignment expression is the entire array 1, 2, 3, regardless of how much or little of it was assigned off. It's the whole thing that was subject to assignment, which is why then that temp ends up pointing at the whole array 1, 2, 3.

[00:06:38] I would say I more typically will put, if I need to capture and also destructure I will more typically put it in this particular order. Meaning that I put the temp there, but there is a little caveat and we'll see that when we talk object destructuring, where it can be convenient to do it the other way around.

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JavaScript Declarations and Assignments

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• var foo [= value [, foo2 [, foo3 ... [, fooN]]]];
• let bar [= value [, bar2 [, foo3 ... [, barN]]]];
• const baz = value [, baz2 = value2 [, ... [, bazN = valueN]]];

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Tech Differences

Know the Technical Differences

Difference Between Definition and Declaration

The definition is automatically a declaration in most of the scenario. Now let’s understand the difference between definition and declaration with the detailed comparison chart.

Content: Definition Vs Declaration

Comparison chart.

• Key Differences

Definition of Definition

Definition identifies the code or data associated with the name of the variable, function, class, etcetera. The definition is necessarily required by the compiler to allocate the storage space for the declared entity. When a variable is defined it holds an amount of memory consist of several bytes for that variable. A function definition produces code for the function. We can define a program element just once in a program because the definition is a unique specification of a program element. The relationship between declaration and definition can be one-to-many .

In some situations, a program element cannot be defined but declared, for example when a function is never invoked or its address is never used even if it is declared. Another example is that in which the class definition is not used while it must be declared.

Definition of Declaration

Declaration is used to specify the names to the program such as the name of a variable, function, namespace, classes, etc. No name can be used in a program without its declaration. The program elements can be declared multiple times, unlike definition. Multiple declarations can only be achieved when the different declarations are made using the identical format. Declaration is the medium of providing visibility to the program element in the perspective of compilers.

The declaration serves the purpose of definition, only in certain cases the condition is not implied which are given below.

• When the static data member is declared inside a class declaration, in that case, it is not a declaration. Because it generates only one copy for all objects of the class and static data members are the components of the objects of a provided class type.
• If a declaration is typedef statement.
• A variable is declared without initializer or function body but includes extern specifiers. It indicates that the definition could be for the other function and provides the name external linkage.
• The class name declaration without including definition such as class T;

Usually, declaration takes place in a scope . The scope decides the visibility of the name declared and the defined object duration.

Key Differences Between Definition and Declaration

• The definition of a program element determines the value associated with that element. On the other hand, the declaration of a program element specifies its name and type to the compiler.
• The definition of the program element reserves some amount of memory while declaration doesn’t involve memory allocation.
• A program element can be declared multiple times. Conversely, definition incorporates a unique specification with the name of the program element which could be distinguished by any code or data.
• Scope in declaration describes the visibility of the variable, function, object, class, enumeration, etc. In contrast, in the definition the scope relates to the duration.

Definition Example

• Variable definition as well as the declaration: int r = 10;
• Function definition: int add (int x , int y) { int a; a = x + y; return a; }

Declaration Example

• Variable declaration: extern int r;
• Function declaration: int add (int p1, int p2);

The declaration process is used to make the program element visible to the compiler, and it doesn’t require to allocate the memory. Inversely definition is a declaration that reserve storage, in simple words the compiler reserves the memory space for the declared entity.

Related Differences:

• Difference Between Stack and Queue
• Difference Between One-Dimensional (1D) and Two-Dimensional (2D) Array
• Difference Between Array and Linked List
• Difference Between Local and Global Variable
• Difference Between Pointer and Reference

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Difference between Definition and Declaration

Declaration of a variable is for informing the compiler of the following information: name of the variable and the type of the value it will hold i.e., declaration gives details about the properties of a variable. Whereas, in the Definition of a variable, memory is allocated for the variable.

In C language definition and declaration for a variable takes place at the same time. i.e. there is no difference between declaration and definition. For example, consider the following statement,

Here, the information such as the variable name: a, and data type: int, is sent to the compiler which will be stored in the data structure known as the symbol table. Along with this, a memory of size 4 bytes(depending upon the type of compiler) will be allocated. Suppose, if we want to only declare variables and not define them i.e. we do not want to allocate memory, then the following declaration can be used

In this example, only the information about the variable name and type is sent and no memory allocation is done. The above information tells the compiler that the variable a is declared now while memory for it will be defined later in the same file or in a different file.

Declaration of a function provides the compiler with the name of the function, the number and type of arguments it takes, and its return type. For example, consider the following code,

Here, a function named add is declared with 2 arguments of type int and return type int. Memory will not be allocated at this stage.

Definition of the function is used for allocating memory for the function. For example, consider the following function definition,

During this function definition, the memory for the function add will be allocated. A variable or a function can be declared any number of times but, it can be defined only once.

The above points are summarized in the following table as follows: 

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