Leonard Martin lennym

What's wrong with Netmag's "Optimize your JavaScript" post

I tweeted earlier that this should be retracted. Generally, these performance-related articles are essentially little more than linkbait -- there are perhaps an infinite number of things you should do to improve a page's performance before worrying about the purported perf hit of multiplication vs. division -- but this post went further than most in this genre: it offered patently inaccurate and misleading advice.

Here are a few examples, assembled by some experts in the field (largely Rick Waldron and Ben Alman, with some help from myself and several others from the place that shall be unnamed).

Factual inaccuracies

Calling array.push() five times in a row will never be a "performance improvement." The author has clearly confused creating an array literal `["foo", "

	$.expr[':'].onscreen = function(obj){
	var $win = $(window), pos = $(obj).position();
	return $.contains(document.documentElement, obj) && pos.top < $win.height() && pos.left < $win.width();
	};

	$.expr[':'].offscreen = function(obj){
	return !$(obj).is(':onscreen');
	};

	Array.prototype.joinProperty = function(delim,method) {
	var output = '';

	for(var i=0; i < this.length; i++){
	if(this[i].hasOwnProperty(method)){
	output = output.concat(this[i][method],delim);
	}else if(eval("typeof this[i]."+method+" != 'undefined'")){
	output = output.concat(eval('this[i].'+method+'()'),delim);
	}
	}

	Array.prototype.joinProperty = function(method, delim) {
	var output = [];
	for(var i=0; i < this.length; i++){
	if(typeof this[i][method] == 'function'){
	output.push(this[i][method]());
	} else {
	output.push(this[i][method]);
	}
	}
	return output.join(delim);

	var vm = require('vm');

	function checkArr(arr) {
	return arr instanceof Array;
	}

	var context = vm.createContext({
	checkArr: checkArr,
	console: console
	});

	[
	{
	"userName": "alphagov",
	"repo": "datainsight-frontend"
	},
	{
	"userName": "alphagov",
	"repo": "fourth-wall"
	},
	{