how to evaluate implicit (unnamed) variable in for loop?

Question

The following works fine in JS:

function GetArray() { return ["x","y","z"]; }
var tmp = GetArray();
var s = "";
for (var i in tmp) { s += i+" = "+tmp[i]+"\n"; }
alert(s);

Which gives:

0 = x
1 = y
2 = z

Now instead of storing GetArray()'s result in a temporary variable, is there also a way to do this by directly evaluating GetArray in the for (...) part?

I mean like this:

for (var i in GetArray()) { /* ??? */ }

Except in this case I don't know how to get element [i] of the array, as there is no explicit variable name to refer to?

Obviously I don't do GetArray()[i] as then GetArray is reprocessed every for-step, which is inefficient and its result may even change (not in this simplified example, but in general).

In PHP there is the concept of foreach(SomeExpression() as $index => $value), is there an equivalent in JS?

If you only care about extra lines of code, then you can always do `for (var i in (tmp = GetArray())) { ... }` — Aleks G, Oct 14 '14 at 11:05
or you use https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach — john Smith, Oct 14 '14 at 11:06
the next version of javascript is going to have a for...of that would solve this same problem more elegantly.https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of — Prabhu Murthy, Oct 14 '14 at 11:12
Thanks to all, works like a charm! And the topic mentioned by Graeme Wicksted really covers it well. — RocketNuts, Oct 14 '14 at 11:23

CodingIntrigue · Answer 1 · 2014-10-14T12:26:39.780

2

You could use forEach:

var s = "";
GetArray().forEach(function(value, index) {
    s += index+" = "+value+"\n";
});

Or reduce:

var s = GetArray().reduce(function(previousValue, currentValue, index) {
    return (index == 1 ? getLine(0, previousValue) : previousValue) + getLine(index, currentValue);
});
function getLine(index, value) {
    return index +" = "+value+"\n";
}

edited Oct 14 '14 at 12:26

answered Oct 14 '14 at 11:06

CodingIntrigue

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score 2 · Accepted Answer · edited May 23 '17 at 12:20

2

arr.forEach(callback[, thisArg])

Gives you:

function callback(element, index, array) {
    // ...
}

BUT the best answer including all caveats can be found here: https://stackoverflow.com/a/9329476/675561

edited May 23 '17 at 12:20

Community

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answered Oct 14 '14 at 11:13

Graeme Wicksted

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And here are some performance statistics on the different for loops: http://jsperf.com/for-vs-foreach/37 – Ole Borgersen Oct 14 '14 at 11:16

wizcabbit · Answer 3 · 2014-10-14T11:23:16.613

2

Answer is YES, if you do not use any library such as jQuery or Underscore, you can use forEach:

GetArray().forEach(function(value, key) {
    // blabla
});

You can find other replacement in Underscore library like:

_.each(GetArray(), function(value, key) {
    // blabla
});

edited Oct 14 '14 at 11:23

answered Oct 14 '14 at 11:17

wizcabbit

36
3

how to evaluate implicit (unnamed) variable in for loop?

3 Answers3