JS: Function (basics)

By Xah Lee. Date: . Last updated: .

Here are the basic ways to define a function.

Arrow function

syntax:

(parameters) => expr

and

(parameters) => {statements}

const ff = (x, y) => {
 return x + y;
};
console.log(ff(3, 4));
// 7

Return statement optional. If none, return undefined.

const ff = (x) => {
 // when no return statement, return undefined
 console.log("good morn");
};
console.log(ff(3));
// good morn
// undefined

curly brackets {} optional if body is a single expression. no return statement.

const ff = (x, y) => x + y;
console.log(ff(3, 4));
// 7

The function keyword

Function declaration

function ff(x) {
 return "Hello " + x;
}
console.log(ff("Alice"));
// Hello Alice

Function expression by the keyword “function”

the function keyword definition can be assigned to a variable.

const ff = function (x) {
 return "Hello " + x;
};
console.log(ff("Alice"));
// Hello Alice

Named function expression by the keyword “function”

// name gg here is allowed but ignored
const ff = function gg(x) {
 if (x < 9) return ff(x + 1);
 else return x;
};

console.log(ff(3));
// 9

console.log(typeof ff);
// function

console.log(typeof gg);
// undefined

🟢 tip: it's good idea to always use Arrow Function. Because defining functions by the function (keyword) have very complex rules of: