Array Comprehensions

What is Array Comprehension?

Array comprehension is creating a list B based on another list A, where each entry of B is the result of a custom operation performed on the items of list A.

Comprehension prevents us from using manual iteration which is error prone and tedious.

JavaScript(ES5) already has similar built-in map and filter functions.

Map & Filter

Best way to understand Array comprehensions is by comparing it with map and filter functions.

Array.prototype.map

This built-in function returns a new array which is the result of applying a callback function.

[1, 2, 3].map(function (i) {
  return i * i;
}); // [1, 4, 9]

[100,012,8000].map(String).join('-'); // "100-012-8000"

We could create an array with Number in-built function.

Array.apply(null, Array(5))
     .map(Function.prototype.call.bind(Number)); // [0, 1, 2, 3, 4]

Array.apply(null, {length: 5})
     .map(Number.call, Number); // [0, 1, 2, 3, 4]

Array.prototype.filter

This function lets you include or exclude some entries of the array based on some certain criteria.

[1,0,2,4,-8,6].filter(function(i) {
  return i < 3
}); // [1, 0, 2, -8]

Filters in practice

Practical/real world implementations of filters can be – find the students who have passed a course.

Array Comprehension in ES6

Part of Firefox but not available in ES5, array comprehensation is now officially a part of ES6.

Easiest way to understand Array Comprehension is by comparing it with map and filter.

Let’s take the previous map example.

[1, 2, 3].map(function (i) {
  return i * i;
}); // [1, 4, 9]

[for(i of [1, 2, 3]) i * i]; // [1, 4, 9]

Here array comprehension uses of clause to iterate the sequence of the array.

Filtering using array comprehension is straightforward.

[1,0,2,4,-8,6].filter(function(i) {
  return i < 3
}); // [1, 0, 2, -8]

[for (i of [1,0,2,4,-8,6]) if(i < 3)];

As you can see, if operates as the filter to produce the sequence we need.

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