rowid,title,contents,year,author,author_slug,published,url,topic 326,Don't be eval(),"JavaScript is an interpreted language, and like so many of its peers it includes the all powerful eval() function. eval() takes a string and executes it as if it were regular JavaScript code. It’s incredibly powerful and incredibly easy to abuse in ways that make your code slower and harder to maintain. As a general rule, if you’re using eval() there’s probably something wrong with your design. Common mistakes Here’s the classic misuse of eval(). You have a JavaScript object, foo, and you want to access a property on it – but you don’t know the name of the property until runtime. Here’s how NOT to do it: var property = 'bar'; var value = eval('foo.' + property); Yes it will work, but every time that piece of code runs JavaScript will have to kick back in to interpreter mode, slowing down your app. It’s also dirt ugly. Here’s the right way of doing the above: var property = 'bar'; var value = foo[property]; In JavaScript, square brackets act as an alternative to lookups using a dot. The only difference is that square bracket syntax expects a string. Security issues In any programming language you should be extremely cautious of executing code from an untrusted source. The same is true for JavaScript – you should be extremely cautious of running eval() against any code that may have been tampered with – for example, strings taken from the page query string. Executing untrusted code can leave you vulnerable to cross-site scripting attacks. What’s it good for? Some programmers say that eval() is B.A.D. – Broken As Designed – and should be removed from the language. However, there are some places in which it can dramatically simplify your code. A great example is for use with XMLHttpRequest, a component of the set of tools more popularly known as Ajax. XMLHttpRequest lets you make a call back to the server from JavaScript without refreshing the whole page. A simple way of using this is to have the server return JavaScript code which is then passed to eval(). Here is a simple function for doing exactly that – it takes the URL to some JavaScript code (or a server-side script that produces JavaScript) and loads and executes that code using XMLHttpRequest and eval(). function evalRequest(url) { var xmlhttp = new XMLHttpRequest(); xmlhttp.onreadystatechange = function() { if (xmlhttp.readyState==4 && xmlhttp.status==200) { eval(xmlhttp.responseText); } } xmlhttp.open(""GET"", url, true); xmlhttp.send(null); } If you want this to work with Internet Explorer you’ll need to include this compatibility patch.",2005,Simon Willison,simonwillison,2005-12-07T00:00:00+00:00,https://24ways.org/2005/dont-be-eval/,code 168,Unobtrusively Mapping Microformats with jQuery,"Microformats are everywhere. You can’t shake an electronic stick these days without accidentally poking a microformat-enabled site, and many developers use microformats as a matter of course. And why not? After all, why invent your own class names when you can re-use pre-defined ones that give your site extra functionality for free? Nevertheless, while it’s good to know that users of tools such as Tails and Operator will derive added value from your shiny semantics, it’s nice to be able to reuse that effort in your own code. We’re going to build a map of some of my favourite restaurants in Brighton. Fitting with the principles of unobtrusive JavaScript, we’ll start with a semantically marked up list of restaurants, then use JavaScript to add the map, look up the restaurant locations and plot them as markers. We’ll be using a couple of powerful tools. The first is jQuery, a JavaScript library that is ideally suited for unobtrusive scripting. jQuery allows us to manipulate elements on the page based on their CSS selector, which makes it easy to extract information from microformats. The second is Mapstraction, introduced here by Andrew Turner a few days ago. We’ll be using Google Maps in the background, but Mapstraction makes it easy to change to a different provider if we want to later. Getting Started We’ll start off with a simple collection of microformatted restaurant details, representing my seven favourite restaurants in Brighton. The full, unstyled list can be seen in restaurants-plain.html. Each restaurant listing looks like this:
12b Meeting House Lane
Brighton, UK
BN1 1HB
Telephone: +44 (0)1273 323 008
E-mail: info@riddleandfinns.co.uk
22-23 Sydney Street
Brighton, UK
BN1 4EN
Telephone: +44 (0)1273 687 068
Lat/Lon: 50.827917, -0.137764