let, const, and var in JavaScript
By: Stella in Javascript Tutorials on 2023-04-25
In JavaScript, let, const, and var are used for declaring variables, but they have different scopes and behaviors. Here are the main differences:
var: This keyword has function scope, which means that the variable declared withvarcan be accessed within the function in which it was defined. If a variable is declared withvaroutside of any function, it will be a global variable and can be accessed throughout the code.varcan be re-declared and updated.
Example:
function foo() {
var x = 5;
if (true) {
var x = 10;
console.log(x); // Output: 10
}
console.log(x); // Output: 10
}
let: This keyword has block scope, which means that the variable declared withletcan be accessed only within the block in which it was defined (for example, within anifstatement,forloop, or function).letcan be updated but not re-declared.
Example:
function foo() {
let x = 5;
if (true) {
let x = 10;
console.log(x); // Output: 10
}
console.log(x); // Output: 5
}
const: This keyword also has block scope, but it is used for declaring constants. The value of aconstvariable cannot be changed or re-declared.
Example:
function foo() {
const x = 5;
if (true) {
const x = 10;
console.log(x); // Output: 10
}
console.log(x); // Output: 5
}
In general, it is recommended to use const for values that will not be changed, let for values that will be changed, and var for global variables. However, the choice of which keyword to use ultimately depends on the specific needs of the code and the programmer's preference.
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Comments (1)
var declarations can have surprising behaviors (for example, they are not block-scoped), and they are discouraged in modern JavaScript code.
If you declare a variable without assigning any value to it, its value is undefined. You can't declare a const variable without an initializer, because you can't change it later anyway.