The Difference Between Let, Var, and Const in JavaScript
Introduction
JavaScript is a dynamically typed language that eliminates the need for defining data types. The language offers different ways of declaring variables, namely let, var, and const. They seem quite similar, but there are differences between the three. In this article, we will explore the differences between let, var, and const in JavaScript.
var vs. let
In older versions of JavaScript, programmers used var to declare variables. The biggest difference between var and let is that the latter is limited in scope to the block, statement, or expression in which it’s used. On the other hand, var has a more extensive scope, as it’s created globally, unless it’s confined to a particular function.
The example below should help you understand how this works:
“`
var x = “hello”
if (true) {
var x = “world” // This overrides x from the outside scope
console.log(x) // Outputs “world”
}
console.log(x) // Outputs “world”
“`
As you can see, using var inside a block can override the variable value outside the block. A good reason to use let is to prevent unintended changes like these. For example:
“`
let x = “hello”
if (true) {
let y = “world”
console.log(y) // Outputs “world”
}
console.log(x) // Outputs “hello”
“`
Now, the variable y is limited in scope to the block, and var outside it isn’t overwritten.
let vs. const
The main difference between let and const is that let allows reassignment while const doesn’t. Once the value is assigned to a const variable, it cannot be changed.
“`
let x = “hello”
x = “world” // No problem
console.log(x) // Outputs “world”
const y = “hello”
y = “world” // SyntaxError
console.log(y)
“`
As you can see, trying to change the const variable throws a syntax error. Another key difference is that const is also block-scoped like let.
Conclusion
In conclusion, let, var, and const are different ways of declaring variables in JavaScript. Choosing the right variable type depends on the use case. If you need to change the value of the variable later, use let. If you know that the value of the variable will remain the same, use const. If you need global scope or function scope, use var. It’s essential to understand the differences between them to write more efficient and safe code.
Table difference between let and var and const in javascript
| Keyword | Scope | Reassignment | Hoisting |
|---|---|---|---|
| var | Function | Allowed | Hoisted |
| let | Block | Allowed | Not Hoisted |
| const | Block | Not Allowed | Not Hoisted |