Array reduce()

JavaScript Array reduce() Definition

The reduce () method reduces the array to a single value. executes a function provided from left to right for each value of the array. reduce() does not execute the function for array elements without values.

Syntax:

array.reduce(function(total, currentValue, currentIndex, arr), initialValue)

total(required): The initialValue, or the previously returned value of the function.
currentValue(required): The value of the current element
currentIndex(optional): The array index of the current element
arr(optional): The array object the current element belongs to

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JavaScript Array reduce() Examples

Example 1

        let names = ["john", "alex", "oliver"];

        let process = names.reduce(function (x, y) {
            return x + y.length;
        },0)
        
        console.log(process)

output:

14

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Example 2

        var numbers = [15, 10, 5, 2];

        function getSum(total, num) {
            return total + Math.round(num);
        }

        console.log(numbers.reduce(getSum,0))

output:

32

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Example 3

        var users = [{
            name: 'Anna',
            password: ['anna123']
        }, {
            name: 'Bob',
            password: ['bob123']
        }, {
            name: 'Alice',
            password: ['alice123']
        }];

        // allbooks - list which will contain all friends' books +  
        // additional list contained in initialValue
        var userProcess = users.reduce(function (x, y) {
            return [...x, ...y.password];
        }, ['password']);

        console.log(userProcess)

output:

[“password”, “anna123”, “bob123”, “alice123”]

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Polyfill

if (!Array.prototype.reduce) {
  Object.defineProperty(Array.prototype, 'reduce', {
    value: function(callback /*, initialValue*/) {
      if (this === null) {
        throw new TypeError( 'Array.prototype.reduce ' + 
          'called on null or undefined' );
      }
      if (typeof callback !== 'function') {
        throw new TypeError( callback +
          ' is not a function');
      }

      // 1. Let O be ? ToObject(this value).
      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0; 

      // Steps 3, 4, 5, 6, 7      
      var k = 0; 
      var value;

      if (arguments.length >= 2) {
        value = arguments[1];
      } else {
        while (k < len && !(k in o)) {
          k++; 
        }

        // 3. If len is 0 and initialValue is not present,
        //    throw a TypeError exception.
        if (k >= len) {
          throw new TypeError( 'Reduce of empty array ' +
            'with no initial value' );
        }
        value = o[k++];
      }

      // 8. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kPresent be ? HasProperty(O, Pk).
        // c. If kPresent is true, then
        //    i.  Let kValue be ? Get(O, Pk).
        //    ii. Let accumulator be ? Call(
        //          callbackfn, undefined,
        //          « accumulator, kValue, k, O »).
        if (k in o) {
          value = callback(value, o[k], k, o);
        }

        // d. Increase k by 1.      
        k++;
      }

      // 9. Return accumulator.
      return value;
    }
  });
}

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Browser Support

Chrome	yes
Edge	yes
Firefox	3
Internet Explorer	9
Opera	10.5
Safari	4
Android webview	yes
Chrome for Android	yes
Edge mobile	yes
Firefox for Android	4
Opera Android	yes