var at循环的最佳用法

Question

I often end up doing something like this:

var IDs = getArrayOfIDs();

IDs.forEach(function(ID){
    var object = myObjects[ID];
    // do stuff with object
});

As far as I understand, this creates a private variable called object for every ID inside my IDs array. Fine.

But I could also do it this way:

var IDs = Obj.getArray();
var object;

IDs.forEach(function(ID){
    object = myObjects[ID];
    // do stuff with object
});

Now I am using a global variable inside my parent function. And with that I don't need to create a new variable for every ID.

Is this an actual advantage (let's say I got a very big array) or a bad idea in general?

Answer 1

Do what is most meaningful and doesn't have a possibility of screwing your code over with problematic variable scopes. If object is only used inside the callback, then create it inside the callback and limit its scope to it. If you don't need it outside, don't make it available outside. By your logic you'd be working with all global variables, and that certainly isn't a good idea.

And with that: I don't need to create a new variable for every ID.

And you aren't actually creating "a new variable". You only have var object once in your code; that's one variable . This variable may be recreated/overwritten a number of times, but it's one logical variable. Let the Javascript engine worry about performance optimization unless you have a very clear indicator that something is an actual performance problem.

Answer 2

What are you optimizing for?

The downsides are:

• looking further up the scope stack for a variable is a tiny bit slower than looking up a local variable
• since the variables will be shared, if you need to use it in a function inside the loop closure, you could start having some issues
• if the variable does not live longer than what you're seeing it will be garbage collected right away.

Also, depending on what the object entry contains, you might just be creating a reference to the variable.