1. What is Hoisting?

Hoisting is JavaScript's default engine behavior of allocating memory space for variable and function declarations during Phase 1 (Memory Creation) before running Phase 2 (Code Execution).

2. Why It Matters

Because memory is created before execution starts, function declarations can be safely invoked before their line of appearance in the source code!

3. Hoisting Behavior: Functions vs. Variables

• Function Declarations: Stored in memory with their full function body during Phase 1. You can call them anywhere in their scope.
• var Variables: Initialized with undefined during Phase 1. Accessing a var variable before its assignment line returns undefined (not a crash!).
• let and const: Memory is allocated during Phase 1, but they remain uninitialized in the Temporal Dead Zone (TDZ). Accessing them before initialization throws a ReferenceError!

// Function hoisting (Works!)
sayHello(); // Output: "Hello World!"

function sayHello() {
  console.log("Hello World!");
}

// Var hoisting (Returns undefined)
console.log(userAge); // Output: undefined
var userAge = 25;

// Let hoisting (Throws ReferenceError!)
// console.log(userName); // ReferenceError: Cannot access 'userName' before initialization
let userName = "Alex";

4. Function Expressions vs. Declarations

Function expressions assigned to variables (var greet = function() {}) hoist as variables, NOT as functions! Calling greet() before assignment throws TypeError: greet is not a function.

// Uncaught TypeError: greet is not a function
// (because 'greet' is currently 'undefined' during execution line 1!)
greet();

var greet = function() {
  console.log("Hi!");
};

5. Common Mistakes

• Calling Arrow Functions Before Declaration: Arrow functions assigned to const or let reside in the TDZ and cannot be called before their declaration line.
• Relying on var Hoisting: Relying on var hoisting creates confusing code where variables are read before assignment. Declare variables at the top of their block.

6. Try It Yourself

Write it yourself. The tests below run against your code.

function getWelcomeMessage(name) {
  const greeting = "Welcome, " + user + "!";
  var user = name;
  return greeting;
}

7. Next Steps

Use modern arrow functions or declare functions clearly before invocation. Next, let's explore concise ES6 Arrow Functions!