What is hoisting in JavaScript?

Short Interview Answer

Hoisting is the behavior where declarations are processed during the creation phase of an execution context before code runs, so bindings exist before their line in source.

Detailed Explanation

During context creation, the engine registers declarations: function declarations are initialized fully; `var` is registered and set to `undefined`; `let`/`const`/class are registered but left in the TDZ. People summarize this as 'declarations are moved to the top,' but nothing is rewritten — it is environment setup. Understanding hoisting prevents bugs with use-before-define and explains differences between declaration kinds.

Example

sayHi();
function sayHi() {
  console.log('hi');
}

Interview Tip

Prefer the execution-context explanation over the 'code is moved' metaphor.

Common Mistake

Saying assignments are hoisted — only the declaration binding is; `var x = 5` still assigns at that line.