What is the difference between "let" and "var" in JavaScript?

Understanding the Scope and Behavior Differences Between let and var

In JavaScript, both var and let are used to declare variables, but they differ in how they handle scoping, redeclaration, and variable hoisting. The introduction of let in ES6 (ECMAScript 2015) was part of a broader effort to address longstanding quirks associated with var and to encourage cleaner, more predictable code.

Key Differences

1. Scope

   • var: Variables declared with var are function-scoped. If declared outside any function, they become globally scoped. This means that if you declare a var variable inside a block (e.g., inside an if statement), it’s still accessible outside that block.
   • let: Variables declared with let are block-scoped. They only exist within the nearest set of braces, such as inside a function, if statement, or for loop. This leads to more predictable and self-contained code blocks.

2. Redeclaration

   • var: You can redeclare the same variable multiple times without an error, which can lead to accidental overwriting of variables.
   • let: You cannot redeclare a variable using let within the same scope. Attempting to do so results in a SyntaxError, helping prevent unintentional variable shadowing or overwriting.

3. Hoisting

   • var: var declarations are hoisted to the top of their function scope, but not their initializations. This means you can reference a var variable before its line of declaration without throwing a ReferenceError, although its value will be undefined until the declaration line is reached.
   • let: let declarations are also hoisted to the top of their block scope, but remain in a “temporal dead zone” (TDZ) until their actual declaration line is executed. Accessing a let variable before it’s declared results in a ReferenceError, ensuring clearer and more predictable behavior.

Example

var Example:

console.log(x); // undefined (due to hoisting)
var x = 10;
if (true) {
  var y = 20;
}
console.log(y); // 20, accessible outside the if-block

let Example:

console.log(a); // ReferenceError: Cannot access 'a' before initialization
let a = 10;
if (true) {
  let b = 20;
}
console.log(b); // ReferenceError: b is not defined (b is block-scoped)

Modern Best Practices

Since let provides block scope and more predictable behavior, it’s generally preferred over var in modern JavaScript code. const—another ES6 addition—goes one step further by creating block-scoped variables that cannot be reassigned, promoting even safer and more stable code.

Strengthen Your JavaScript Foundations

To confidently navigate JavaScript features like let and var—and to fully understand fundamental concepts such as scoping, hoisting, and variable lifecycles—consider building a strong foundation in the language:

• Grokking JavaScript Fundamentals: Perfect for beginners or those looking to refresh their knowledge, this course ensures you understand modern JavaScript best practices, language patterns, and core principles. By mastering these essentials, you’ll write more robust, maintainable, and future-proof code.

In Summary

• Scope: var is function-scoped, while let is block-scoped.
• Redeclaration: var allows redeclaration; let does not.
• Hoisting: Both are hoisted, but let enforces a temporal dead zone, preventing access before declaration.

Embrace let (and const) for more predictable, cleaner, and more maintainable code as you refine your JavaScript expertise.