tl;dr.
The filter function is used to select specific items from the original array based on a condition, while the map function is used to create a new array by applying a transformation to each item of the original array.
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Array.prototype.filter
The filter() function calls a predicate function on every element and filters out the elements that pass a test.
let nums = [0, 1, 2, 3, 4, 5, 6] nums.filter((x) => x * 2) // [1, 2, 3, 4, 5, 6]
In this example, the predicate function x * 2 is used to determine whether each item should be filtered or not. In JavaScript, non-zero values are considered true. As a result, all elements except 0 have passed the test and been included in the filtered array.
Array.prototype.map
The map() function creates a new array by applying a callback function to each element of the original array, and returns the resulting array.
let nums = [0, 1, 2, 3, 4, 5, 6] nums.map((x) => x * 2) // [0, 2, 4, 6, 8, 10, 12];
The map(callback) function executes the callback function on each item of the array and stores the returned values in a new array. The function continues this process until all items have been processed, and finally returns the new array.