Using AJAX/JSON and performance issues with eval()

Discussion in 'Javascript' started by bizt, Jul 1, 2008.

  1. bizt

    bizt Guest


    Im currently looking to move into using JSON for AJAX instead of
    returning from the server a string like the following:

    12345{This is a text string{true[1234|This is another set of fields|

    This is returned from the server and I use split() and loops to
    generate JS arrays / nested arrays at the client end. I cannot use XML
    is it is deemed to heavy for the website as we deal with very high
    volumes of traffic and trying to reduce the data sent back and forth
    through the client and servers. The problem with the above is that it
    makes my life as a JS programmer harder coz I have to do all this
    splitting and looping to seperate the data

    Now JSON seems the perfect on both fronts - it is lightweight and I
    only have to use a single eval() to replace what splits() and loops
    done previously. I can also have as many levels and nesting as I wish.
    There are a couple of conserns I have here tho:

    Eval() I have heard can be quite slow? Is this a serious concern for
    large data sets? Even if it is slow, is it still going to be faster
    than my previous method above?

    I understand that you can have a JSON format for JS arrays and JS

    // array
    {0: 12345,2:"This is a text string"}

    // object
    [12345,"This is a text string"]

    I prefer objects because I dont need to specifiy the index key and I
    can treat it exactly like an array - oSet[0] = ...
    Are objects just as quick to access/alter as arrays? I have a few
    simultaneous client processes going on (ie. refreshing multiple HTML
    tables automatically in the background) and really want it to perform

    Is there anything else worth considering when using JSON? My website
    is mainly just updating content on multiple tables and refreshing data
    every so often, thats about the extent of it.


    bizt, Jul 1, 2008
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  2. I don't think so.
    Probably yes. eval() evaluates its string argument as an ECMAScript
    Program. This evaluation is implemented in native, already compiled (with
    few exceptions platform-dependent) code. That should be considerably faster
    to execute than your implementation, which needs to be JIT-compiled and the
    resulting byte-code interpreted by a VM first.
    Just to add to confusion: JS arrays are implemented as objects, Array
    objects. What you call "JS objects" here, are (augmented) Object objects.
    The above is the reason for that. `0' is the name of either object's
    property, only that the method of property access with Array objects differs
    slightly from that of other objects.
    I think they are quicker to alter than arrays when it comes to inserting
    items, and just as quick when accessing items. To be sure, you could do an
    estimation of general runtime efficiency on the corresponding algorithms in
    the ECMAScript Specification, Edition 3 Final, and compare with empirical
    results for different input.
    When dealing with tables, you should consider a combination of both:

    {x: 42, z: 23},
    {x: 1337, z: 1701}


    Please don't use off-Usenetter slang like "coz" here. Besides presenting
    you as a semi-literate, this is an international newsgroup.
    Thomas 'PointedEars' Lahn, Jul 1, 2008
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