Javascript - Access elements of array using an array of indices

Question

Is there a shortcut to accessing elements of an array using an array of indices rather than going one index at a time?

Example (this doesn't work):

var array = ["One", "Two", "Three", "Four"];
var indices = [1, 3];
var result = array[indices];

where result would be ["Two", "Four"].

• Comfortably Numb: you can extend array with a function that would have this functionality

Answer 1

You can make one and have it available to all Arrays if you've no qualms about extending native prototypes in your environment.

Array.prototype.atIndices = function(ind) {
 var result = [];
 for (var i = 0; i < arguments.length; i++) {
 if (arguments[i] in this)
 result.push(this[arguments[i]])
 }
 return result; 
}

---

var result = array.atIndices(1,3);

---

You could also have it check to see if an Array was passed, or a mix of indices and Arrays.

Array.prototype.atIndices = function(ind) {
 var result = [];
 for (var i = 0; i < arguments.length; i++) {
 if (Array.isArray(arguments[i]))
 result.push.apply(result, this.atIndices.apply(this, arguments[i]))
 else if (arguments[i] in this)
 result.push(this[arguments[i]])
 }
 return result; 
}

---

This will actually flatten out all Arrays, so they could be as deeply nested as you want.

var result = array.atIndices(1, [3, [5]]);

• plalx: Note that you don't have to extend Array.prototype and you probably shouldn't. Just makes the function signature to take the array as argument.
• user2437417: @plalx: There's really no should or shouldn't. If building a public library it's probably not the best idea, but other than that, it depends entirely on the decisions made about the development environment by the individual or team.
• plalx: Well most of todays libraries assumes that prototypes aren't modified. In my opinion you should avoid modifying native object prototypes, even if you fully control your development environment, unless it will never be opened to any third-party libs and even then, the API simplification is such as small benefit. array.atIndices(1) is not that much simpler than someNS.atIndices(array, 1).
• user2437417: "...most of todays libraries assumes that prototypes aren't modified" Care to share your data? And note that there's quite a big difference between extending Array.prototype and Object.prototype.
• plalx: I just made up that statement =P However, What if a future ecma's spec eventually includes an Array.prototype.atIndices function that behaves differently? That's just one of many cases that could lead to an unpredictable maintenance nightmare. Here's a good article by Nicholas C.Zakas that might change your mind nczonline.net/blog/2010/03/02/…

Answer 2

Now there is:

function pluck(arr, indices) {
 var result = [],
 i = 0,
 len = indices.length;
 for (; i < len; i++) {
 result.push(arr[indices[i]]);
 }
 return result;
}

Answer 3

As an alternative to rolling your own, if you have access to Lo-Dash (which you should totally use because it is awesome) its at function does exactly what you want.

Your usage would be:

var result = _.at(array, indices);

See: http://lodash.com/docs#at