reflect-args

This npm module defines a means of retrieving parameters from a function from outside the function. It works with any function or closure, including methods, but not with constructors wrapped inside ES6 classes.

Installation

$ npm install reflect-args

Usage

Before using it, we need to define it:

const getArgs = require('reflect-args').getArgs

The getArgs function takes in a function and returns a Map containing all the parameters of that function.

Function or Closure

let func = function (foo, bar = 12, test = '12', cb = () => {}) {
    // TODO: body stub
}
 
console.log(getArgs(func))
 
/* Expected output:
 * Map {
 *     'foo' => undefined, 
 *     'bar' => 12, 
 *     'test' => '12',
 *     'cb' => [Function: cb]
 * }
 */

Method (Can Be Static)

let Class = class Test
{
    constructor () {}
    
    func (foo, bar) {
        
    }
}
let inst = new Class()
 
console.log(getArgs(inst.func))
 
/* Expected output:
 * Map {
 *     'foo' => undefined, 
 *     'bar' => undefined	
 * }
 */

Pattern Matched Parameters

Sometimes you may want to let the end user (programmer) use pattern matching inside their function whose arguments are reflected. During such cases, the getArgs function will give the keys the names of an incrementing range:

let patternMatched = function ({foo, bar}, test, [more, and])
{
    
}
 
console.log(getArgs(patternMatched))
 
/* Expected output:
 * Map {
 *     '__0' => '{foo, bar}', 
 *     'test' => undefined, 
 *     '__1' => '[more, and]'
 * }
 */

Discussion

The way this works is by utilizing the Function.prototype.toString function, by extracing the arguments from that string. This means that if the code is obfuscated, this will not work.

That same sentence holds true for reflection done in any other language. If the code is obfuscated, the reflected variable names will be changed into whatsoever they were obfuscated into.

As for most cases, there is no point in minifying (thus obfuscating) server-side code, as we do not actually have to send the data to some client, thus saving on band-width. If, for some reason, you would like to minify server-sided code using this module, make sure the minifier does not obfuscate away variable names.