JS: Function Parameters

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);

Parameter Default Value

• JS: Function Parameter Default Value

Rest Parameters

• JS: Function Rest Parameters

Function Argument Destructure

• JS: Function Argument Destructure (Pattern Matching)

the 「arguments」 Object

• JS: the arguments object

How to find the number of arguments passed?

The total count of arguments passed is arguments.length.

• JS: the arguments object

How to find out how many parameters are required?

• JS: Function Length Property