Question

I am stuck using an AJAX library from about 5 years ago in this project, and it had some issues with parsing dates in JSON. I wound up rewriting its parse function to use a single regex:

return eval('(' + (enableDateParsing ? text.replace(/"(?:\\)?\/Date\((.*?)\)(?:\\)?\/"/g, "new Date($1)") : text) + ')');

This works really well, but I thought I could get a speed up if I used native JSON parsing in IE8 / chrome / ff, so I added this bit:

if (typeof JSON !== 'undefined' && typeof JSON.parse !== 'undefined') {     
        var nativeJsonDateParseRegex = /\/Date\(.*?\)\//g;       
        return JSON.parse(text, function (key, value) {
            if (AjaxPro.enableDateParsing && typeof value === 'string' && value.match(nativeJsonDateParseRegex)) 
            {
                value = new Date(parseInt(value.substr(6)));
            }                       
            return value;         
        });        
    } 
else // revert to eval for ie6/ie7

The reviver callback will execute once for each JSON property returned, so it has to be very fast. During a profile I’ve seen it’s been called 170484 times, but still runs pretty fast (131.237ms). Any ideas on how to make it faster, or is this the best you can do without serious tweaking?

Answer 1

Your code contains a lot of constant conditions, you’ll be fine with checking once whether native JSON is supported or not.

Suggestions:

• check for native JSPN support at page load, and add the right function accordingly.
• Drop the global flag from the regex if you do not need it
• Drop regular expressions if possible, if every date always starts with “/Date(“, search for it. It’s much faster (see benchmark at jsperf.com)
• todo: check whether parseInt can be replaced with an other method to get rid of the trailing )/.
• If AjaxPro.enableDateParsing is a constant, you can remove if from AjaxPro.jsonParse and and make it a condition like the check for native JSON

Code without RE:

if (typeof JSON !== 'undefined' && typeof JSON.parse !== 'undefined') {
    AjaxPro.nativeJsonDateParseRegex = /\/Date\(.*?\)\//g;
    AjaxPro.dateFunc = function(key, value) {
       if (typeof value === "string" && !value.indexOf("/Date(")) {
           return new Date(value.substring(6, value.length-2));
       }
       return value;
    };
    AjaxPro.jsonParse = function(text) {
        if (AjaxPro.enableDateParsing) {
            return JSON.parse(text, AjaxPro.dateFunc);
        }
        return JSON.parse(text);
    };
} else // revert to eval for ie6/ie7

This should be highly optimized. You might want to run some more test on your own in multiple browsers. Maybe checking for a property of a string is faster than checking its type (doubt it), thing like that.