Arguments
You can call JavaScript functions with fewer or more arguments than the number they expect!
Fewer Arguments
Missing argument? Parameter is undefined:
This feature can be used to create optional parameters; check for undefined parameters and assign them a default value.
In modern JavaScript, you can use default parameter values by assigning a value to a parameter in the function signature.
function average(x, y = x) {
return (x + y) / 2;
}
Multiple default arguments ok:
More Arguments
If you provide a function with more arguments than it expects, it will ignore the extra ones!
Rest Parameters/Spread Operator
We've seen the "rest parameters" in the destructuring assignment with arrays and objects. You can use the same pattern to capture a variable number of arguments:
JavaScript has a "spear operator" which looks identical to the "rest operator" but its purpose is to expand (spread out) the elements of an iterable object.
const numbers = [3, 4, 5]
const result = total(...numbers);
You can create a function that takes both named parameters and rest parameter but in that case the rest parameter must be the last parameter.
function myFunction (arg1, arg2, ...rest) { /* do something awesome with all the arguments! */ }
Arguments Object
If you are puzzled by how functions can be called with fewer/more arguments, here is an explanation. It turns out, the named parameters of a function are more like guidelines than anything else. Every function has access to an array-like object called arguments. This object holds all of the values passed to the function.
If you declare a function with n named parameters, JavaScript will assign the first n elements of arguments to those parameters, in order.
Caution: Arrow function do no have
argumnetsobject.