Variables in JavaScript

What are Variables in JavaScript and the Best 25 Examples

In the world of programming, data processing is a fundamental aspect of applications. In information processing, one of the important elements in all programming languages, including JavaScript, are variables. These valuable tools allow users and programmers to store and manipulate data that is essential to the functionality of applications.

In the upcoming article from “thinkwriteway.com blog”, the variable definition in JavaScript, the types of variables in JavaScript, as well as the appropriate techniques for declaring and using them will be discussed. In addition, this article covers basic concepts such as variable domains in JavaScript.

Variable definition in JavaScript

In JavaScript, different keywords are used to declare or define a variable, each of which has a specific purpose. The most important of these keywords are the following:

JavaScript let, JavaScript var, JavaScript const

  • let: In modern JavaScript, this keyword is used to declare variables. When using let, the variable can be reassigned and this feature allows flexibility in the data it holds.
  • var: This keyword represents an older approach to declaring variables in JavaScript. Variables declared with JavaScript var are also reassignable, but in terms of scope, they behave differently, which we will discuss further.
  • const: Similar let, const It is used to declare variables but with a significant difference. When a value to the variable const is allocated, it cannot be changed during program execution. This immutability is useful when the user wants to ensure that the variable retains its initial value throughout the code.
variables in Javascript

By understanding the differences between these methods of variable definition in JavaScript, developers can make informed decisions about which type of variable declaration is best suited to their specific use cases.

How many ways are there to define variables in JavaScript?

Normally, the variable definition in JavaScript is done using various keywords such as “Var”, “Let” and “Const”, but there are other ways to do this which are equally important and include the “declaration” method. function”, “class declaration” and “import command”. It is important to pay attention to the following explanations regarding these methods.

  • Function declaration: In JavaScript, functions can be defined using the keyword function announced Functions can have parameters and can also return values. Variables inside functions using JavaScript var, let, or const are declared, they are considered local to the scope of that function.
  • Class declaration: JavaScript ES6 introduced classes as a new way to create objects. keyword class It allows developers to define classes that can contain properties and methods. Variables declared in a class using JavaScript var, let, or const Class members are considered.
  • IMPORT Statement: With the advent of ES6 modules, JavaScript now supports importing variables from other files. Phrase import Used to import variables or functions declared in another module and make them accessible in the current module.

In the next sections of this article, in addition to variable definition in JavaScript, using keywords var, let, and const the methods mentioned above will be taught in full along with examples. Before dealing with these variable declaration methods in JavaScript, it is useful to review the rules for naming variables.

Variable naming rules in JavaScript

Like any other language, JavaScript has its own rules for defining variables. In JavaScript, variable names must obey the following two specific restrictions:

  • Variable names contain only letters, numbers or symbols $ And _ Is.
  • The first character must not be one.

Choosing appropriate and meaningful variable names is a vital skill in programming. Well-named variables increase code readability and maintainability. It is better for users to spend enough time on the naming process before declaring the names of the variables. When using multiple words in a variable name, it is recommended to follow the “camelCase” syntax, where the first word begins with a lowercase letter and subsequent words begin with an uppercase letter. for example, randomLongName Named by this contract.

It is interesting to know that the same sign $ And the underline _ can be used in variable names. They are treated as regular symbols, just like letters, without any special meaning. As such, the following variables are all validly named.

let $ = 10; // variable with the name "$"
let _ = 25; // variable with the name "_"

console.log($ + _); // Output: 35

Also, the following examples have wrong names:

let 1a; // Variable names cannot start with a digit
let my-name; // Hyphens '-' aren't allowed in variable names

On the other hand, certain words are already reserved in the language and cannot be used as variable names because they are already used by the language itself. Some examples of reserved words are: let, class, return, function Etc. In the article JavaScript var from thinkwriteway.com blog a list of these words is given. Attempting to use them as variable names will result in syntax errors. As in the following example:

let let = 50; // Error: "let" cannot be used as a variable name
let return = 25; // Error: "return" cannot be used as a variable name

In short, by choosing descriptive and appropriate variable names that adhere to naming conventions, we have more readable, more maintainable code that includes better programming.

Definition Variable in JavaScript Var

keyword var It used to be used to declare variables in JavaScript and allow reassignment of their values. However, this keyword for variable definition in JavaScript contains the following problems:

” Hoisting in JavaScript “: Variables declared with var They are placed at the top of their range and are accessible before they are actually announced. As a result, variables declared with this method until the appropriate value is assigned to it, value undefined they keep

Restatement: In the same scope, the use of var to redeclare a variable essentially reassigns its value, which leads to unintended consequences.

variables in Javascript

Lack of “block scope” (Block Scope): The variable declared with var JavaScript does not have a block-level scope, and this creates potential problems in managing variables in larger codebases.

Due to these shortcomings, the use of var is no longer common in modern JavaScript development. Instead, now developers let and const are preferred for variable declarations as they provide better range and help avoid Hoisting-related bugs. The following is an example of how to use JavaScript var shows to define the variable:

console.log(x); // Output: undefined
var x = 1;


var x = 2;
console.log(x); // Output: 2

To increase code quality and avoid such problems, it is better than let and constUsed to define variables in modern JavaScript projects.

let in JavaScript to define a variable

In JavaScript, the keyword let Used to declare variables that can be reassigned. Unlike var Variables declared with let cannot be used before their declarations and avoid any issues with Hosting in JavaScript. In addition, let It does not allow to declaration of the same variable in the same scope, which solves the problem of random variable assignment. How to create or define a variable in JavaScript using the keyword let is as follows:

let information;

Once the variable is declared, the data can be assigned using the assignment operator (=) attributed to the variable.

let information = "Hello you";

Also, multiple variables can be declared in a few lines:

let user = 'John';
let age = 25;
let message = 'Hello my Friend';

In this method of variable definition in JavaScript, trying to declare the same variable more than once causes an error. A variable must be declared only once in the same scope:

let information = 'coco banjo';
// Repeated 'let' leads to an error
let information = 'baba banjo'; // SyntaxError: 'information' has already been declared

It should be noted that when using let to define a variable in JavaScript, in order to avoid errors, avoid declaring the variable more than once in a certain range. One of the significant benefits let This is what introduces the block range. Variables declared with let They are limited to the block in which they are defined, such as loops, conditional statements, or function bodies. This helps to manage variables in JavaScript more effectively and reduces the possibility of unwanted side effects.

Variable definition with Const in JavaScript

In JavaScript, the keyword const, is used to declare static variables, which are basically variables that cannot be reassigned after initial allocation. Once a value is assigned to a constant, any attempt to change it will result in an error, as the following example illustrates.

const myAddress = 'Sherbrooke, Canada';

In the example above, myAddress A type is a constant and cannot be assigned a new value later.

By using const in JavaScript, programmers can explicitly state that a variable is intended to remain unchanged during program execution. This method of variable definition in JavaScript is especially useful when the user wants the value of the variable to be known and unchanged before executing the code. Another common use const Create aliases for values ​​that are hard to remember. Constants, in this context, are often named using capital letters and underlining, which makes their names easier to remember. The following example is important to understand this concept:

const COLOR_BLUE = "#00F";
const COLOR_ORANGE = "#FF7F00";

let color = COLOR_ORANGE;
console.log(color); // Output: #FF7F00
variables in Javascript

In the above example it is acceptable to use the “CAPITAL_LETTERS” naming convention for constant names, but note that this practice usually applies to constants with values ​​that are known before the code is executed. Constants whose values ​​are determined at runtime can be named using the “camelCase” naming convention, as the following example illustrates.

const pageLoadTime = /* time taken by a webpage to load */;

In the above example, pageLoadTime is still considered a constant, even if its value is determined at runtime.

Using functions to define variables in Javascript

In JavaScript, a function declaration allows the user to create a variable and assign a function to it. Below is an example of a function declaration with this method:

function sum(a, b) {
  return a + b;
}

The function declaration is similar to variable declaration with let and the assignment of the function expression is named:

let sum = function sum(a, b) {
  return a + b;
};

One important aspect of function declarations is that they are placed above their “Function Scope” or “Module Scope”. This means that the function can be called before it is actually declared in the code. The following example illustrates this concept:

console.log(sum(2, 3)); // Output: 5
function sum(a, b) {
  return a + b;
}
variables in Javascript

Unlike the similarities that this method has with the variable definition in JavaScript let Yes, the function declaration allows the variable to be reassigned to any other value, including the new function expression.

function sum(a, b) {
  return a + b;
}
sum = (a, b) => a - b;
console.log(sum(1, 2)); // Output: -1

It’s important to note that while this flexibility can be useful, there are cases where it would be better if the function declaration created a static function and prevented accidental reassignment. In general, the function declaration in JavaScript allows the user to create variables that hold functions. They are placed at the top of their range and like variables let, they can be assigned to other values. However, in some situations, it may be preferable to treat functions as constants.

Use JavaScript class to define a variable

In JavaScript, the class declaration allows the user to create a variable and assign a class to it. The following example is given to understand this concept.

class Todo {
  constructor(title) {
    this.title = title;
    this.done = false;
  }
}

This is similar to variable definition in JavaScript let and the assignment of the class expression will be with the name, which is an example as follows:

let Todo = class Todo {
  constructor(name) {
    this.name = name;
  }
};

Function and class declarations are placed above their functional or module scope, meaning they can be used before they are actually declared in code. The following example illustrates this concept:

console.log(new Todo("task")); // Output: Todo { name: "task" }
class Todo {
  constructor(title) {
    this.title = title;
    this.done = false;
  }
}

Class declaration, such as declarations let, allows variable reassignment. This means that the user can save the class in different ways or even reassign it to the new class expression, as the following example shows:

class Todo {
  constructor(title) {
    this.title = title;
    this.done = false;
  }
}
Todo = class {
  constructor(name) {
    this.name = name;
  }
};
console.log(new Todo("task")); // Output: Todo { name: "task" }

In the code above, the variable Todo First, will store a class that is an object with two properties title and done creates After reassignment, it holds the new class variable, which is an object with the single attribute name Creates. It is worth noting that the variable Todo can be different values, even null, reassigned. The following example illustrates this:

Todo = null;
console.log(Todo); // Output: null

As with function declarations, this flexibility can be useful, but there are still cases where it’s better to declare a static creation class to avoid accidental reassignment. In general, it can be said that the class declaration in JavaScript allows the programmer to create a variable that holds this class variable. such as variables let, class declarations can be assigned to other values.

Import command for variable definition in JavaScript

The import command in JavaScript will act as a unique way to define a variable in JavaScript that receives its value from another module. In JavaScript, a module is basically a type of file. For example, below and in the file variable.js A variable is entered:

let x = 1;
export default x;

In the next example, create a constant with a value from the module variable.js is initialized. As opposed to variable x in the module variable.js With let It will be prompted for reallocation, which will become a constant when it is imported into the new module:

import x from './variable';
// Error: "x" is read-only.

When the variable with command import is entered, x will become a constant and cannot be assigned in the new module:

import x from './variable';
x = 2; // Error: "x" is read-only.

In this case, trying to reallocate x to a new value will result in an error because the expression import considers it as fixed. In short, the phrase import to define a variable in JavaScript, allows the user to declare a constant that receives its value from another module. Once imported, the variable becomes immutable and cannot be assigned in the new module. This feature ensures the integrity and consistency of values ​​obtained from external modules.

Tips for defining variables in JavaScript

In this section, there are key points about variable definition in JavaScript and other important concepts related to them, which will remind users to define their variables properly.

  • keywords let and const the recommended method for declaring variables is JavaScript var It is still available, but has issues and is not used in modern JavaScript development.
  • Function declarations create a variable and assign a function to it. Similarly, class declarations also create a variable and assign it a class.
  • Variables created using the keyword’s function and class can be assigned to new functions or classes.
  • Phrase import Allows the user to create a constant type that receives its value from another module. Once imported, the constant becomes immutable and cannot be reassigned.

What is the scope of variables in Javascript?

Variables in JavaScript have different scopes or domains that determine their access. In general, the following three types of scopes are available for JavaScript variables, which are “Block Scope,” “Function Scope,” and “Global Scope.” The scope of variables in JavaScript refers to the context in which a variable is defined and can be accessed. Each of these domains will be explained in the following part of the variable definition tutorial in JavaScript. Also, to better understand the concept of scope in JavaScript, it is recommended to read the article “Scope in JavaScript.”

Block scope of variables in JavaScript

Variables declared with let and const are block-scoped, meaning that they are accessible only within the block (part of the code between {} ) in which they are declared. The scope of variables in JavaScript is restricted to the specific block they are defined in. The following example is given to understand the block range:

if (true) {
   let y = 15;
   var x = 40;
   console.log(x); // Output: 40
   console.log(y); // Output: 15
}

console.log(x); // Output: 40
console.log(y); // Error: y is not defined

For the above example, if the browser console is opened, we will see the following result:

variables in Javascript

Note: Variables declared with JavaScript var They have no block scope and will be accessible from outside the block in which they are defined.

Functional scope of variables in JavaScript

Variables declared inside a JavaScript function are function-scoped, meaning they are local to that function and cannot be accessed outside of it. This is an important aspect of the scope of variables in JavaScript. The following example is given to understand this concept:

function randomFunction() {
   let barName = "Irish Pub";
   console.log(barName); // Output: Irish Pub
}

console.log(barName); // Error: barName is not defined

Global scope of variables in JavaScript

Variables declared outside any function or block have global scope and can be accessed from anywhere in the JavaScript program. The following example helps a lot to understand the concept of global scope:

let carName = "Tesla";

function carFunction() {
   console.log(carName); // Output: Tesla
}

console.log(carName); // Output: Tesla

It is important to understand the scope of variables in JavaScript. Variables in JavaScript have a lifecycle, as they are created when declared and exist until the end of their scope. Function (local) variables are deleted after the function completes, while global variables remain until the browser window or tab in the web browser is closed. Function arguments (parameters) also act as local variables within functions.

In summary, JavaScript variables can be block, function, or global in scope, depending on how and where they are declared. Proper understanding and use of variable scopes are essential for writing efficient and bug-free code.

Conclusion

In this article from thinkwriteway.com, how to define variables in JavaScript and their types were taught in a practical way. In this article, we learned that to declare variables in addition to Javascript var, Javascript let, and Javascript const of the other three approaches, function declaration, class declaration, and command Import They are used for this purpose.

If you seek further information about the What are Variables in JavaScript and the Best 25 Examples or have any inquiries regarding our blog, please don’t hesitate to contact us via email at info@thinkwriteway.com

Share This Article :

Leave a Comment

What to read next