如何在javascript中获取for（... in ...）循环的位置？

Question

When looping over a javascript object using a for in loop, how can I access the position of the iterator inside the for loop?

a = {some: 1, thing: 2};

for (obj in a) {
    if (/* How can I access the first iteration*/) {
       // Do something different on the first iteration
    }
    else {
       // Do something
    }
}

Answer 1

A Javascript object's properties have no ordering. {some: 1, thing: 2} is the same as {thing: 2, some: 1}

But if you want to keep track using an iterator anyway, do this:

var i = 0;
for (obj in a) {
    if (i == 0) {
       // Do something different on the first iteration
    }
    else {
       // Do something
    }
    i ++;
}

Answer 2

As far as I am aware there's no innate way to do so, and there's no way to know which is the first item, the order of the properties are arbitrary. If there's something you only want to do once, well, that's incredibly simple, you just keep a manual iterator, but I'm not sure if that's what you're asking for.

a = {some: 1, thing: 2};
var first = true;

for (obj in a) {
    if (first) {
       first = false;
       // Do something different on the first iteration
    }
    else {
       // Do something
    }
}