Parse a JavaScript string function definition and return a function object. Does not use eval.

parseFunction.js

/* Parse a string function definition and return a function object. Does not use eval.
 * @param {string} str
 * @return {function}
 *
 * Example:
 *  var f = function (x, y) { return x * y; };
 *  var g = parseFunction(f.toString());
 *  g(33, 3); //=> 99
 */
function parseFunction (str) {
  var fn_body_idx = str.indexOf('{'),
      fn_body = str.substring(fn_body_idx+1, str.lastIndexOf('}')),
      fn_declare = str.substring(0, fn_body_idx),
      fn_params = fn_declare.substring(fn_declare.indexOf('(')+1, fn_declare.lastIndexOf(')')),
      args = fn_params.split(',');

  args.push(fn_body);

  function Fn () {
    return Function.apply(this, args);
  }
  Fn.prototype = Function.prototype;
	
  return new Fn();
}

nice, works with lambdas too.

This is probably the most underrated utility on the entire GitHub. Pity that such a helpful utility has got just 9 stars!

Thanks!!!

Thanks for sharing!

I did some modification to support async function:

function parseFunction (str) {
  const is_async = str.trim().startsWith('async'),
      fn_body_idx = str.indexOf('{'),
      fn_body = str.substring(fn_body_idx+1, str.lastIndexOf('}')),
      fn_declare = str.substring(0, fn_body_idx),
      fn_params = fn_declare.substring(fn_declare.indexOf('(')+1, fn_declare.lastIndexOf(')')),
      args = fn_params.split(',');
  args.push(fn_body);
  
  if(is_async){
    const AsyncFunction = Object.getPrototypeOf(async function(){}).constructor;
    function Fn () {
      return AsyncFunction.apply(this, args);
    }
    Fn.prototype = AsyncFunction.prototype;
  }
  else{
    function Fn () {
      return Function.apply(this, args);
    }
    Fn.prototype = Function.prototype;
  }
  return new Fn();
}

Does not work with destructured function parameters (es6) :(