JS: Function Parameters

By Xah Lee. Date: 2014-06-30. Last updated: 2026-02-25.

Number of Arguments Not Checked

for function defined by keyword function, JavaScript does not check the number of arguments passed in a function call.

If there are more arguments passed in a function call than declared parameters, they are ignored.
If less, then extra parameter variables have values of undefined .

// extra arguments are ignored.
function ff(x) {
 return x;
}
console.assert(ff(2, 3, 4) === 2);

// for arrow function too.
const gg = (x) => x;
console.assert(gg(2, 3, 4) === 2);

// unfilled parameters have value of undefined

function ff(x, y) {
 return y;
}

const gg = (x, y) => y;

console.assert(ff(3) === undefined);

console.assert(gg(3) === undefined);