Hoisting in javascript

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope. var declarations are hoisted and initialized with undefined, while let/const are hoisted but stay in the temporal dead zone until declared.

Example

console.log(a); // undefined (var is hoisted)
var a = 10;

// console.log(b); // ReferenceError (temporal dead zone)
let b = 20;

var is hoisted and initialized to undefined, so reading it before assignment does not crash; let/const are not usable before their declaration.

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