首页
学习
活动
专区
工具
TVP
发布
社区首页 >专栏 >JavaScript中如何中断forEach循环

JavaScript中如何中断forEach循环

作者头像
meteoric
发布2018-11-20 11:09:40
2.7K0
发布2018-11-20 11:09:40
举报
文章被收录于专栏:游戏杂谈游戏杂谈

先来看下forEach的实现

// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: http://es5.github.io/#x15.4.4.18
if (!Array.prototype.forEach) {

  Array.prototype.forEach = function(callback, thisArg) {

    var T, k;

    if (this === null) {
      throw new TypeError(' this is null or not defined');
    }

    // 1. Let O be the result of calling toObject() passing the
    // |this| value as the argument.
    var O = Object(this);

    // 2. Let lenValue be the result of calling the Get() internal
    // method of O with the argument "length".
    // 3. Let len be toUint32(lenValue).
    var len = O.length >>> 0;

    // 4. If isCallable(callback) is false, throw a TypeError exception. 
    // See: http://es5.github.com/#x9.11
    if (typeof callback !== "function") {
      throw new TypeError(callback + ' is not a function');
    }

    // 5. If thisArg was supplied, let T be thisArg; else let
    // T be undefined.
    if (arguments.length > 1) {
      T = thisArg;
    }

    // 6. Let k be 0
    k = 0;

    // 7. Repeat, while k < len
    while (k < len) {

      var kValue;

      // a. Let Pk be ToString(k).
      //    This is implicit for LHS operands of the in operator
      // b. Let kPresent be the result of calling the HasProperty
      //    internal method of O with argument Pk.
      //    This step can be combined with c
      // c. If kPresent is true, then
      if (k in O) {

        // i. Let kValue be the result of calling the Get internal
        // method of O with argument Pk.
        kValue = O[k];

        // ii. Call the Call internal method of callback with T as
        // the this value and argument list containing kValue, k, and O.
        callback.call(T, kValue, k, O);
      }
      // d. Increase k by 1.
      k++;
    }
    // 8. return undefined
  };
}

基本用法:

arr.forEach(callback[, thisArg]),callback会接收到三个参数:currentValue、index、array

var ary = ["JavaScript", "Java", "CoffeeScript", "TypeScript"];

ary.forEach(function(value, index, _ary) {
    console.log(index + ": " + value);
    return false;
});

// logs:

0: JavaScript 1: Java 2: CoffeeScript 3: TypeScript

使用some函数

var ary = ["JavaScript", "Java", "CoffeeScript", "TypeScript"];

ary.some(function (value, index, _ary) {
    console.log(index + ": " + value);
    return value === "CoffeeScript";
});

// logs:

0: JavaScript 1: Java 2: CoffeeScript

使用every函数

var ary = ["JavaScript", "Java", "CoffeeScript", "TypeScript"];

ary.every(function(value, index, _ary) {
    console.log(index + ": " + value);
    return value.indexOf("Script") > -1;
});

// logs:

0: JavaScript 1: Java

使用fo..of

let arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
for (let el of arr) {
  console.log(el);
  if (el === 5) {
    break;
  }
}

// logs:

0

1

2

3

4

5

而如果forEach想实现类似every、some函数的效果该如何做呢?

在stackoverflow上得票比较高的有如下几类方法 :

1、循环外使用try.. catch,当需要中断时throw 一个异常,然后catch进行捕获;

2、重写forEach(也是借鉴第一种方法);

var BreakException = {};

try {
  [1, 2, 3].forEach(function(el) {
    console.log(el);
    if (el === 2) throw BreakException;
  });
} catch (e) {
  if (e !== BreakException) throw e;
}
// Use a closure to prevent the global namespace from be polluted.
(function() {
  // Define StopIteration as part of the global scope if it
  // isn't already defined.
  if(typeof StopIteration == "undefined") {
    StopIteration = new Error("StopIteration");
  }

  // The original version of Array.prototype.forEach.
  var oldForEach = Array.prototype.forEach;

  // If forEach actually exists, define forEach so you can
  // break out of it by throwing StopIteration.  Allow
  // other errors will be thrown as normal.
  if(oldForEach) {
    Array.prototype.forEach = function() {
      try {
        oldForEach.apply(this, [].slice.call(arguments, 0));
      }
      catch(e) {
        if(e !== StopIteration) {
          throw e;
        }
      }
    };
  }
})();
  
  
// Show the contents until you get to "2".
[0,1,2,3,4].forEach(function(val) {
  if(val == 2)
    throw StopIteration;
  alert(val);
});

参考链接:

http://dean.edwards.name/weblog/2006/07/enum/

http://www.jsnoob.com/2013/11/26/how-to-break-the-foreach/

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of

本文参与 腾讯云自媒体分享计划,分享自作者个人站点/博客。
原始发表:2016-09-30 ,如有侵权请联系 cloudcommunity@tencent.com 删除

本文分享自 作者个人站点/博客 前往查看

如有侵权,请联系 cloudcommunity@tencent.com 删除。

本文参与 腾讯云自媒体分享计划  ,欢迎热爱写作的你一起参与!

评论
登录后参与评论
0 条评论
热度
最新
推荐阅读
领券
问题归档专栏文章快讯文章归档关键词归档开发者手册归档开发者手册 Section 归档