JavaScript let

ES6 introduced the let keyword to declare variables for all types of data like boolean, object, integer, etc.

In my last blog, I discussed the introduction to variables in JavaScript, hoisting, and the var keyword.

The let

• The let keyword declares a block-scoped variable in Javascript.
• Same variable can't be declared again but can be initialized again.
• Variable declared with let are hoisted and but remain uninitialized.

1. Syntax

let x = 5;
console.log(x); //5

let y;
y = false;
console.log(y); //false

2. Block scoping

variables declared with let create block scoping.

Example-1:

let range = 10;
if (range > 10) {
  let food = "Pizza";
  console.log(food); //Pizza
}
console.log(food); // ReferenceError: food is not defined

Here let created a block scope. That's why the food variable is only accessible inside the if-statement not outside of it.

Example-2:

for(let i = 0; i < 5; i++) {
    // some task
}
console.log(i); // ReferenceError: i is not defined

Due to block scope, the i variable is only accessible inside the for-loop not outside of it.

3. Redeclaration and re-initialization

When using let we can't re-declare variables. JavaScript throws the Syntax Error. Only one declaration for one variable but variables can be re-initialized with any value.

Re-declaration

let a = true;
let a = false;
console.log(a); 
// SyntaxError: Identifier 'a' has already been declared

Re-initialization

let a = true;
a = false;
console.log(a); // false

If we only declare variable with let and not assign a value, then Javascript assigns undefined to it. Later, when code execution reaches the variable assignment new values replace the undefined value.

let a;
console.log(a); // undefined

a = 10;
console.log(a); // 10

4. Hoisting

let declarations are hoisted but they are not initialized at the compile time. Due to this if we try to access a variable before it was declared, Javascript throws us an error.

Example:

str = "hello";
let str;
// ReferenceError: Cannot access 'str' before initialization

Explanation: When our code is compiled let declarations are hoisted but they are not initialized. They will only get initialized when during runtime code execution reaches the assignment. So, we can't access the variable before it was declared in code.

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