// 最基礎的forEach function forEach(array, action) { for (var i = 0; i < array.length; i++) { action(array[i]); } } // 測試forEach forEach(["Pear", "Apple"], function(name) { console.log(name); }); // ------------------------------------------------ // // 實現reduce function reduce(combine, base, array) { forEach(array, function(element) { base = combine(base, element); }); return base; } // 使用reduce執行個體1:計算數組中的0的個數 function countZeros(array) { function counter(total, elem) { return total + (elem == 0 ? 1 : 0); } return reduce(counter, 0, array); } alert("countZeros by reduce: " + countZeros([1, 3, 0, 4, 7, 0])); // 使用reduce執行個體2:求和 function sum(array) { function add(a, b) { return a + b; } return reduce(add, 0, array); } alert("sum by reduce: " + sum([1, 2, 3, 5])); // ------------------------------------------------ // // 實現map function map(func, array) { var result = []; forEach(array, function(elem) { result.push(func(elem)); // 對於map,func函數一般只有一個參數,所以用func(elem) }); return result; } // 利用map實現數組的每個數字翻倍 var array = [1, 2, 3, 4, 5]; var mappedArray = map(function(elem) { return elem * 2; }, array); console.log(mappedArray); // 利用map實現數組向下取整 var array2 = [1.3, 4.5, 6.7, 8, 9.2]; var mappedArray2 = map(Math.floor, array2); console.log(mappedArray2); // ------------------------------------------------ // // 實現filter: function filter(func, array) { var result = []; forEach(array, function(elem) { if(func(elem)) result.push(elem); }); return result; } // 使用filter過濾出偶數 function isEven(elem) { return elem % 2 == 0; } var array3 = [1, 2, 3, 4, 5, 6, 7, 8, 9]; var filterArray = filter(isEven, array3); console.log(filterArray);