ECMAS: Array,prototype.reduce()

柔情只为你懂 2021-09-29 08:06 229阅读 0赞

The reduce() method executes a reducer function (that you provide) on each member of the array resulting in a single output value. [for details][]

## Syntax ##

> arr.reduce(callback\[, initialValue\])

sectionParameters:

*  callback 接受4个参数
    
     *  accumulator
     *  currentValue
     *  currentIndexOptional
     *  arrayOptional
 *  initialValueOptional

return:

The value that results from the reduction

## 案例 ##

*  数组元素求和

var sum = [0, 1, 2, 3].reduce(function (a, b) {
        console.log('time')
        return a + b;
    }, 0);
    // sum is 6
    // 需要注意的是这里循环了4次

*  将二维数组转化为一维

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
      function(a, b) {
        return a.concat(b);
      },
      []
    );
    // flattened is [0, 1, 2, 3, 4, 5]

*  计算数组中每个元素出现的次数

*  数组去重

let arr = [1,2,1,2,3,5,4,5,3,4,4,4,4];
    let result = arr.sort().reduce((init, current)=>{
        if(init.length===0 || init[init.length-1]!==current){
            init.push(current);
        }
        return init;
    }, []);
    console.log(result); //[1,2,3,4,5]

## Polyfill ##

// Production steps of ECMA-262, Edition 5, 15.4.4.21
    // Reference: http://es5.github.io/#x15.4.4.21
    // https://tc39.github.io/ecma262/#sec-array.prototype.reduce
    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;
        }
      });
    }

转载于:https://www.cnblogs.com/gaollard/p/9678881.html

[for details]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce