function - Counting parameters in Javascript -

what docs @ mozilla says:

  console.log((function(...args) {}).length);    // 0, rest parameter not counted    console.log((function(a, b = 1, c) {}).length);   // 1, parameters before first 1    // default value counted 

so how ever able count parameters in such cases ?

https://developer.mozilla.org/en-us/docs/web/javascript/reference/global_objects/function/length

there 2 separate things going on here parameters defined , arguments passed.

for defined function, can access number of parameters defined in definition of function fn.length:

function talk(greeting, delay) {     // code here }  console.log(talk.length);     // shows 2 because there 2 defined parameters in                                // function definition 

separately, within function, can see how many arguments passed function given invocation of function using arguments.length. suppose had function written accept optional callback last argument:

function talk(greeting, delay, callback) {     console.log(arguments.length);     // shows how many arguments passed }  talk("hello", 200);    // cause function show 2 arguments passed talk("hello", 200, function() {    // show 3 arguments passed      console.log("talk done now"); });