javascript - Filter array based on an array of index

Question

First I apologize if it's a duplicate (I searched but did not find this simple example...), but I want to select elements of arr1 based on the index in arr2:

arr1 = [33,66,77,8,99]
arr2 = [2,0,3] 

I am using underscore.js but the 0 index is not retrieved (seems to be considered as false):

res = _.filter(arr1, function(value, index){
    if(_.contains(arr2, index)){
        return index;
    }
});

Which returns:

# [77, 8]

How could I fix this, and is there a simpler way to filter using an array of indexes? I am expecting the following result:

# [77, 33, 8]

Answer 1

The simplest way is to use _.map on arr2, like this

console.log(_.map(arr2, function (item) {
  return arr1[item];
}));
// [ 77, 33, 8 ]

Here, we iterate the indexes and fetching the corresponding values from arr1 and creating a new array.

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Equivalent to the above, but perhaps a bit more advanced, is to use _.propertyOf instead of the anonymous function:

console.log(_.map(arr2, _.propertyOf(arr1)));
// [ 77, 33, 8 ]

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If your environment supports ECMA Script 6's Arrow functions, then you can also do

console.log(_.map(arr2, (item) => arr1[item]));
// [ 77, 33, 8 ]

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Moreover, you can use the native Array.protoype.map itself, if your target environment supports them, like this

console.log(arr2.map((item) => arr1[item]));
// [ 77, 33, 8 ]