How to get function parameter names/values dynamically?

Question

Is there a way to get the function parameter names of a function dynamically?

Let’s say my function looks like this:

function doSomething(param1, par         


        

Answer 1

I have read most of the answers here, and I would like to add my one-liner.

new RegExp('(?:'+Function.name+'\\s*|^)\\((.*?)\\)').exec(Function.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '')

or

function getParameters(func) {
  return new RegExp('(?:'+func.name+'\\s*|^)\\s*\\((.*?)\\)').exec(func.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '');
}

or for a one-liner function in ECMA6

var getParameters = func => new RegExp('(?:'+func.name+'\\s*|^)\\s*\\((.*?)\\)').exec(func.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '');

__

Let's say you have a function

function foo(abc, def, ghi, jkl) {
  //code
}

The below code will return "abc,def,ghi,jkl"

That code will also work with the setup of a function that Camilo Martin gave:

function  (  A,  b
,c      ,d
){}

Also with Bubersson's comment on Jack Allan's answer:

function(a /* fooled you)*/,b){}

__

Explanation

new RegExp('(?:'+Function.name+'\\s*|^)\\s*\\((.*?)\\)')

This creates a Regular Expression with the new RegExp('(?:'+Function.name+'\\s*|^)\\s*\\((.*?)\\)'). I have to use new RegExp because I am injecting a variable (Function.name, the name of the function being targeted) into the RegExp.

Example If the function name is "foo" (function foo()), the RegExp will be /foo\s*\((.*?)\)/.

Function.toString().replace(/\n/g, '')

Then it converts the entire function into a string, and removes all newlines. Removing newlines helps with the function setup Camilo Martin gave.

.exec(...)[1]

This is the RegExp.prototype.exec function. It basically matches the Regular Exponent (new RegExp()) into the String (Function.toString()). Then the [1] will return the first Capture Group found in the Regular Exponent ((.*?)).

.replace(/\/\*.*?\*\//g, '').replace(/ /g, '')

This will remove every comment inside /* and */, and remove all spaces.

---

This also now supports reading and understanding arrow (=>) functions, such as f = (a, b) => void 0;, in which Function.toString() would return (a, b) => void 0 instead of the normal function's function f(a, b) { return void 0; }. The original regular expression would have thrown an error in its confusion, but is now accounted for.

The change was from new RegExp(Function.name+'\\s*\\((.*?)\\)') (/Function\s*\((.*?)\)/) to new RegExp('(?:'+Function.name+'\\s*|^)\\((.*?)\\)') (/(?:Function\s*|^)\((.*?)\)/)

---

If you want to make all the parameters into an Array instead of a String separated by commas, at the end just add .split(',').