Simon Willison’s Weblog

Don’t be eval()

7th December 2005

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.

This article originally appeared on 24ways.