{"rowid": 326, "title": "Don't be eval()", "contents": "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\u2019s incredibly powerful and incredibly easy to abuse in ways that make your code slower and harder to maintain. As a general rule, if you\u2019re using eval() there\u2019s probably something wrong with your design.\n\nCommon mistakes\n\nHere\u2019s the classic misuse of eval(). You have a JavaScript object, foo, and you want to access a property on it \u2013 but you don\u2019t know the name of the property until runtime. Here\u2019s how NOT to do it:\n\nvar property = 'bar';\nvar value = eval('foo.' + property);\n\nYes 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\u2019s also dirt ugly.\n\nHere\u2019s the right way of doing the above:\n\nvar property = 'bar';\nvar value = foo[property];\n\nIn JavaScript, square brackets act as an alternative to lookups using a dot. The only difference is that square bracket syntax expects a string.\n\nSecurity issues\n\nIn any programming language you should be extremely cautious of executing code from an untrusted source. The same is true for JavaScript \u2013 you should be extremely cautious of running eval() against any code that may have been tampered with \u2013 for example, strings taken from the page query string. Executing untrusted code can leave you vulnerable to cross-site scripting attacks.\n\nWhat\u2019s it good for?\n\nSome programmers say that eval() is B.A.D. \u2013 Broken As Designed \u2013 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 \u2013 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().\n\nfunction evalRequest(url) {\n     var xmlhttp = new XMLHttpRequest();\n     xmlhttp.onreadystatechange = function() {\n          if (xmlhttp.readyState==4 && xmlhttp.status==200) {\n               eval(xmlhttp.responseText);\n          }\n     }\n     xmlhttp.open(\"GET\", url, true);\n     xmlhttp.send(null);\n }\n\nIf you want this to work with Internet Explorer you\u2019ll need to include this compatibility patch.", "year": "2005", "author": "Simon Willison", "author_slug": "simonwillison", "published": "2005-12-07T00:00:00+00:00", "url": "https://24ways.org/2005/dont-be-eval/", "topic": "code"}