如何使以下代码更快？

Question

I am developing a application with phonegap. on my pc everything runs fine but on my mobile device its just too slow.

i think the problem is on the show function, the android browser seems to needs really long to hide and show elements

what can be improved?

function show(id){
    $('.view').hide()
    //alert('show ' + id)
    $('#'+id+'View').show()
    scroll(0,0)
}


function getSoundHTML(id, myname, del){

    if (del != true){
        var imgsrc = 'plus.png'
        var f = function(){
            addToCostumSounds(id)
            alert('added to favorites')
        }
    }else{
        var imgsrc = 'minus.png'
        var f = function(){
            removeFromCostumSounds(id);
            $(this).fadeOut().next('div').fadeOut();
        }
    }

    var div = $('<div></div>').addClass('box').html(myname).css('border-color', '999999').click(function(){
        play(id)
    })
    var img = $('<img></img>').attr('src', imgsrc).addClass('sideimg').click(f)

    return $('<div></div>').append(img).append(div)
}

for(var category in categories){

    var f = function(category){
        $('#'+category+'Btn').click(function(){
                show(category)

                var categoryView = $('#'+category+'View')
                for(var key in categories[category]){
                    var div = getSoundHTML(key, categories[category][key])
                    categoryView.prepend(div)
                }
                var img = '<img src="menu.png" class="menuimg"/>'
                categoryView.prepend(img)
                categoryView.append(img)
        })
    }
    f(category)
}

the html:

    <div class="btn" id="noBtn">no _</div>
    <div class="btn" id="thatIsBtn">that is _</div>
    <div class="btn" id="thereIsBtn">there is _</div>
    <div class="btn" id="thisIsBtn">this is _</div>
    ...


<div class="view" id="noView"></div>
<div class="view" id="thatIsView"></div>
<div class="view" id="thereIsView"></div>
<div class="view" id="thisIsView"></div>
...

Answer 1

Whilst it may not have an effect on Desktops, your massive lack of semi-colons in the right places may have an effect on mobile devices.

The JavaScript engine has to run through and try to work out where the semi-colons need to go. See this transcript from the ECMAScript specification .

To be honest I think thats only a few milliseconds of time-saved, but its a starting point for now - and good practice for the future.

Answer 2

Here's your problem:

for(var category in categories){

    var f = function(category){
        ...
        for(var key in categories[category])
        ...
    }

    f(category)
}

You have two BIG issues here:

1. You're defining a function within a loop. While this is sometimes needed, you should do your very best to avoid defining things within a loop unless you absolutely need to. In this case, you could probably move the f function out of the loop entirely without breaking your code.
2. Nested loops. You have a for... in within a for... in loop. This is largely due to the first problem I pointed out, but in general nested loops are a big no-no from a performance standpoint.

Answer 3

ok, i think i got the only way to improve the peroformance: if someone clicks on a button (class="btn") he is redirected to a new fresh page that has the entire page in HTML and does not build it with javascript.