Voice of a Developer: JavaScript Array Methods - Part Seventeen

Sumit Jolly
May 09, 2016
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JavaScript is a language of the Web. This series of articles will talk about my observations learned during my decade of software development experience with JavaScript.

Before moving further let us look at the previous articles of the series:

New Array methods were introduced, and widely used now. I find these extremely useful and many modern JS libraries are heavily dependent on these, e.g., UnderscoreJS

We’ll look at common used Array methods

Map

The map() method creates a new array with the results of calling a function for every array element.

Parameters

Callback - produces an element of the new Array & runs over each element

currentValue - current element being processed
index - index of current Array element
array - array map was called upon

var numbers = [1,2,3,4];
var ary1=numbers.map (function (item,index) {
return item *2;
});
console.log(ary1); // [2, 4, 6, 8]

Therefore, the above code leverages Map method & iterates over each item and produces a new Array containing product by 2. Index is optional to pass & you could use inside callback function (item,index)

Reduce

The reduce() method reduces the array to a single value.

Parameters

A callback function runs on each value and takes four arguments:

previousValue - value returned previously
currentValue - current element being processed
index - index of current Array element
array - array map was called upon

Example

var numbers = [1,2,3,4];
var total=numbers.reduce (function (prev,current) {
return prev+current;
});
console.log(total); // 10

Look at stack trace where prev & current values could be debugged. This method processes two values at a time.

code

Filter

It creates a new array with all elements which pass the test.

Parameters

Callback - to test each element of array. It return true to keep the element, false otherwise.

var numbers = [1,2,3,4];
var ary1 = numbers.filter (function (value) {
return value > 2;
});
console.log (ary1); // [3,4]

Find

It returns a value in array if it’s found, otherwise undefined is returned.

Parameters

Callback - to test each element of array.

element - current processed value.
index - index of current Array element.
array - array map was called upon.

var numbers = [1,2,3,4];
var value = numbers.find (function (element) {
return element == 2;
});
console.log(value); // 2

Every

The purpose of this method is to iterate over each element of an Array and validate for all elements. Here is the syntax from VSCode.

code