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Extended answer to StackOvferlow question <a href="http://stackoverflow.com/questions/6921588">Is it possible to reflect the arguments of a javascript function</a>

<pre id="results"></pre>

JavaScript

function test0 (a, b, c) {};

function test1( a, b, c ) {};

function test2() {};
var test3 = function(){};
var test4 = function () {};
var test5 = "sfsf (a, b, c)";

var test6 = function(delay,
    str = ' ',
    str2 = ',',
    json = { foo: 54, bar: true },
    callback) {
    setTimeout(() => {

        callback({ foo: 42, bar: 'Yello timeout!', data: this.options });
    }, delay * 1000);
};

var argRE = /^\s*function\s+(?:\w*\s*)?\((.*?)\)/;

document.getElementById('results').innerHTML = ([test0, test1, test2, test3, test4, test5, test6].map(

function (f, i) {
    return (f.toString() + Array(32).join(' ')).slice(0, 30) +
        ' : <span style="color:green">' + 
        ((f = f.toString().match(argRE)) 
            ? (f[1] ? '[' + f[1].trim().split(/\s*,\s*/) + ']' 
            : '[]') : null) +
        '</span>';
})).join('\n');