javascript - Return every second item from a list with Underscore.js? - Stack Overflow

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I'd like to return every second item from a list, using Underscore.

I guess I could do something like this:

var i = 0;
_.filter([1,2,3,4...], function(item) { 
  i++;
  return (i%2 == 0);
});

but is there a more elegant way, native to Underscore?

I'd like to return every second item from a list, using Underscore.

I guess I could do something like this:

var i = 0;
_.filter([1,2,3,4...], function(item) { 
  i++;
  return (i%2 == 0);
});

but is there a more elegant way, native to Underscore?

• javascript
• underscore.js

Share Improve this question Follow asked Apr 8, 2014 at 16:28 RichardRichard 65.7k135135 gold badges356356 silver badges571571 bronze badges 4

• No need for underscore here... you can simply do [1,2,3,4...].filter(function(_,i){ return i%2;}) it's fully supported inall non-really-ancient browsers (yes, even IE9). – Benjamin Gruenbaum Commented Apr 8, 2014 at 16:33
• i would suggest using function takeEveryN(item, index) {return index % this == 0;}; and calling it like _.filter([1,2,3,4], takeEveryN, 2); so you can hop over different numbers of items later... – dandavis Commented Apr 8, 2014 at 16:36
• @dandavis that's confusing since there already is an Array.prototype.every that does something else. – Benjamin Gruenbaum Commented Apr 8, 2014 at 16:37
• 1 mine's not an [] prop, but i'll concede the point and edit my ment to remove ambiguity. good input. my main point was to use a named function because it encourages reuse and defining functions closer to global makes the code faster to run in V8 if not elsewhere since it doesn't re-parse the function as much... – dandavis Commented Apr 8, 2014 at 16:38

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3 Answers 3

Sorted by: Reset to default 5

The function which is getting called by _.filter will get the current index as well, as the second parameter and you can make use of that like this

console.log(_.filter([1, 2, 3, 4], function(item, index) {
    return index % 2 == 0;
}));
# [ 1, 3 ]

Since you want to start from the second element, you just have to change the condition a little, like this

console.log(_.filter([1, 2, 3, 4], function(item, index) {
    return index % 2 == 1;
}));
# [ 2, 4 ]

Alternatively, you can use the native Array.prototype.filter in the same way, like this

console.log([1, 2, 3, 4].filter(function(item, index) {
    return index % 2 == 1;
}));

_.filter([1,2,3,4...], function(item, index) { 
  return (index % 2 == 0);
});

filter predicate accepts two parameters - item value and index value.

var test = [1,2,3,4];
var tt = _.first(_.rest([5, 4, 3, 2, 1]));
console.log(tt);

Fiddle

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