JavaScript數組對象，有一個forEach方法，可枚舉每一個數組元素，但并不支持類似for循環的break語法，中斷循環：

[1,2,3].forEach(function(item) {  // if(!item) break; 不支持});

解決辦法，可拋出一個特殊異常，來中斷forEach循環，原理：

var BreakException = {};try {[1, 2, 3].forEach(function(el) {console.log(el);if (el === 2) throw BreakException;});} catch (e) {if (e !== BreakException) throw e;}

也可復寫forEach方法：

// 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;}}};}})();

使用

// Show the contents until you get to "2".[0,1,2,3,4].forEach(function(val) {if(val == 2)throw StopIteration;alert(val);});

總結

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