Functions

What is a function?

A function is a reusable block of code that performs a specific task.
It is called to avoid repeating code and improve readability.

In plain English:
➡ “Give a name to a set of instructions, and reuse them whenever you want.”

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Function declaration

function greet(name) {
  return "Hello " + name;
}

console.log(greet("Alice")); // Hello Alice

Here, greet is a declared function.
It can be called before its definition thanks to the hoisting mechanism.

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Function expressions

You can also store a function in a variable.
In this case, it cannot be called before its definition.

const greet = function(name) {
  return "Hi " + name;
};

console.log(greet("Bob")); // Hi Bob

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Arrow functions

Introduced with ES6, they offer a shorter syntax.
They automatically inherit the this of their context (lexical this), which makes them convenient in callbacks.

const greet = (name) => "Hello " + name;

console.log(greet("Bob")); // Hello Bob

Important difference:

• A classic function defines its own this.
• An arrow function uses the this of the context where it was defined.

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Parameters and default values

Parameters can have a default value if no argument is provided.

function multiply(a, b = 2) {
  return a * b;
}

console.log(multiply(5));    // 10
console.log(multiply(5, 3)); // 15

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Returning a value

A function can return a result with return.

function add(x, y) {
  return x + y;
}

let result = add(4, 6);
console.log(result); // 10

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Syntax comparison

Syntax	Example	Particularity
Declaration	function name() {}	Hoisting (can be called before definition)
Expression	const f = function() {}	No hoisting
Arrow (ES6)	const f = () => {}	Short syntax, lexical this

Tip

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