JavaScript: Function Parameters

Number of Arguments Not Checked

JavaScript doesn't 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 .

function ff(x) {
  return x;
}

// extra arguments are ignored
console.log(ff(2, 3, 4) === 2);

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

// extra arguments are ignored
console.log(gg(2, 3, 4) === 2);

// unfilled parameters have value of undefined
function gg(x, y) {
  return y;
}
console.log(gg(3) === undefined);

// unfilled parameters have value of undefined
const gg = ((x, y) => y);
console.log(gg(3) === undefined);

Parameter Default Value

Parameter can have default value, like this:

function f(param=value) {…}

Function Argument Default Value

Rest Parameters

A function can have unspecified number of parameters, like this:

function f (...name) { return name; }

Function Rest Parameters

Function Argument Destructure

Function Argument Destructure

the 「arguments」 Object

arguments Object

console.log(
  (function (x, y) {
    return x + y;
  }).length,
);
// 2