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les.szkla..

JSON Performance comparison of eval and new Function

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in Developing jQuery Core • 12 years ago

Hello,

Why new Function is used in the code below instead of eval?

According to this blog post eval is generally faster.

http://weblogs.asp.net/yuanjian/archive/2009/03/22/json-performance-comparison-of-eval-new-function-and-json.aspx

// Try to use the native JSON parser first
return window.JSON && window.JSON.parse ?
window.JSON.parse( data ) :
(new Function("return " + data))()

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jeresig

Re: JSON Performance comparison of eval and new Function

12 years ago

Using eval causes all sorts of problems for code minifiers since it's not clear what could be executing in the eval. Looking at the last results from that run it looks like new Function is fairly equivalent to eval and even slightly faster sometimes. The one exception was Safari 4 - but those results are dated, Safari 4 shipped with a native JSON.parse implementation, which we use.

curiousda..

Re: Re: JSON Performance comparison of eval and new Function

12 years ago

I have a related question John: MDC says that each time you run a new Function() the code is compiled again, but your Secrets book doesn't mention that at all saying that Function() should be preferred unless you need eval() scope. Obviously this doesn't matter much for JSON when you're only going to run it once, but for general code compilation it does. Is MDC correct and eval the only real option for dynamic compilation?

infinity88

Re: JSON Performance comparison of eval and new Function

12 years ago

The Microsoft Ajax Minifier has a setting to consider Evals as "ignore", or "local". When 'ignore' it means you are assuming the eval is not intended to access any scope, so it can safely minify as usual. Local means it is intended to access the current scope, but ONLY the current scope, so parent scopes can still be minified. Another mode, Safe, means it may access any scope.

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JSON Performance comparison of eval and new Function

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