Hoisting

To hoist means to raise or lift something up, often using ropes, chains, or mechanical tools like a crane

Full Hoisting Example

applies to functions and variables declared with var (not let or const)


saySomething(); // Output: "Hello, world!"
 
function saySomething() {
  console.log("Hello, world!");
}

How JS reads the code:


function saySomething() { // hoisted to the top
  console.log("Hello, world!");
}
saySomething(); // Output: "Hello, world!"


console.log(myVar); // Output: undefined
var myVar = 10;
console.log(myVar); // Output: 10

How JS reads the code:


var myVar; // hoisted to the top; Declared AND assigned "undefined"
console.log(myVar); // Output: undefined
myVar = 10;
console.log(myVar); // Output: 10

Partial Hoisting Example

applies to let and const variables; these are hoisted (moved up) but not initialized (i.e. like var gets initialized with undefined), so they cannot be accessed before their declaration.


console.log(myVar); // Output: ReferenceError: Cannot access 'myVar' before initialization
let myVar = 10;
console.log(myVar); // Output: 10