In JavaScript, `var`, `let`, and `const` are three keywords that are used to declare variables. They have some similarities and some differences, which are described below:
1. `var`
The `var` keyword is used to declare a variable that is either global or local to the current function. Variables declared with `var` are hoisted to the top of the current scope, which means that you can access them before they are declared. However, this can lead to some unexpected behavior, particularly when using `var` inside loops.
console.log(x); // Output: undefined
var x = 10;
console.log(x); // Output: 10
2. `let`
The `let` keyword is used to declare a block-scoped variable. Variables declared with `let` are not hoisted to the top of the current scope, which means that you cannot access them before they are declared. `let` variables can be reassigned, but they cannot be redeclared within the same block.
console.log(x); // Output: ReferenceError: x is not defined
let x = 10;
console.log(x); // Output: 10
x = 20;
console.log(x); // Output: 20
3. `const`
The `const` keyword is used to declare a block-scoped, read-only variable. Variables declared with `const` cannot be reassigned or redeclared within the same block.
console.log(x); // Output: ReferenceError: x is not defined
const x = 10;
console.log(x); // Output: 10
x = 20; // Output: TypeError: Assignment to constant variable.
In general, it is recommended to use `const` by default, and only use `let` if you need to reassign the variable. The `var` keyword should be avoided because it can lead to unexpected behavior and is not as modern as `let` and `const`.
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