What is hoisting in JavaScript?

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Hoisting in JavaScript is a default behavior of the JavaScript interpreter where variable and function declarations are moved (“hoisted”) to the top of their scope (global or function) before code execution.

This means you can use variables or functions before they are declared in the code, though the way it works depends on whether you use var, let, const, or a function declaration.

1. Variable Hoisting

• Variables declared with var are hoisted but initialized as undefined.

• Variables declared with let and const are hoisted too, but they remain in a Temporal Dead Zone (TDZ) until the declaration is reached. Accessing them before declaration throws a ReferenceError.

Example:

console.log(a); // undefined (due to var hoisting)
var a = 10;

console.log(b); // ReferenceError (TDZ)
let b = 20;

2. Function Hoisting

• Function Declarations are fully hoisted, meaning you can call them before their definition.

• Function Expressions (using var, let, or const) are treated like variables, so only the variable name is hoisted, not the function body.

Example:

greet(); // Works because of hoisting
function greet() {
  console.log("Hello!");
}

sayHi(); // TypeError: sayHi is not a function
var sayHi = function() {
  console.log("Hi!");
};

✅ In short:

• Declarations are hoisted, initializations are not.

• var → hoisted as undefined.

• let / const → hoisted but blocked in TDZ.

• Function declarations → fully hoisted.

• Function expressions → behave like variables.

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