What the docs at Mozilla says:
console.log((function(...args) {}).length);
// 0, rest parameter is not counted
console.log((function(a, b = 1, c) {}).length);
// 1, only parameters before the first one with
// a default value is counted
So how will I ever be able to count parameters in such cases ?
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length
There are two separate things going on here with parameters defined and arguments actually passed.
For a defined function, you can get access to the number of parameters that are defined in the definition of the function with fn.length:
function talk(greeting, delay) {
// some code here
}
console.log(talk.length); // shows 2 because there are two defined parameters in
// the function definition
Separately, from within a function, you can see how many arguments are actually passed to a function for a given invocation of that function using arguments.length. Suppose you had a function that was written to accept an optional callback as the last argument:
function talk(greeting, delay, callback) {
console.log(arguments.length); // shows how many arguments were actually passed
}
talk("hello", 200); // will cause the function to show 2 arguments are passed
talk("hello", 200, function() { // will show 3 arguments are passed
console.log("talk is done now");
});
arguments.length gives you the number directly? I suppose you could iterate over the arguments object to see how many properties it has, but I don't see the point when arguments.length already contains the desired count.arguments within the function rather than outside of it.