"Async" loop over an object in javascript

Question

Normally, we can do loops for both arrays and objects to iterate over the properties/values. But loops are blocking. However, timeouts can be used to simulate an async loop. i managed to do it for an array.

//do stuff

(function asyncLoop(i){

    //do stuff in the current iteration

    if(++i < array.length){
        setTimeout(function(){asyncLoop(i);}, 1);
    } else {
        callback();
    }
}(0));

//do stuff immediately after, while looping

but this model only works while looping in an array, where there is a limiter - the i that gets passed around. is there a way to do this over an object? let's just say that the object has 50k keys to iterate through, making it unreasonably long.

i already know of this setImmediate (afaik, only newer IE) and WebWorkers(not yet in IE), but i just want to know if it's possible to just use the same strategy on an object.

jfriend00 · Accepted Answer · 2012-04-18 10:27:54Z

14

There is no async-capable iterator of properties because there's no way to save the state of where you are in the iterator other than the for (key in obj) loop and you already know that isn't async-compatible.

So, just collect all the keys of the object into an array and use the same mechanism you already have to iterate over the array of keys. Arrays have the advantage that they do have a way to save the state of where you are in the iteration by just keeping track of the array index.

One can get all the keys, either with the Object.keys(obj) ES5 method (via built-in methods or ES5 shim if needed) or you can collect them yourself if you aren't otherwise using ES5 shims:

var keys = [];
for (var i in obj) {
    if (obj.hasOwnProperty(i)) {
        keys.push(i);
    }
}

edited Apr 18, 2012 at 10:27

answered Apr 18, 2012 at 10:22

jfriend00

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4 Comments

Joseph Over a year ago

but that would mean looping through the object just to put them (or the references) into the array.

jfriend00 Over a year ago

@Joseph - yes, one would have to first collect the properties into the array. There is no async iterator of properties.

katspaugh Over a year ago

Object.keys + es5-shim

T.J. Crowder Over a year ago

@Joseph: Yes, it does mean looping (whether you do it, or use Object.keys to do it), but even with an object with a truly massive number of properties, it won't take long to build the array. Presumably the reason you want this to be async is that what you're doing with each entry takes time.

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