如何使用map和foreach过滤数组中不在另一个数组中的元素
[英]How to filter elements in an array that are not in another array using map and foreach
问题描述
I want to find the elements in an array that are not present in a second array, using only the .map() and .forEach() methods (without using .filter() or other methods) 
我想仅使用.map()和.forEach()方法（不使用.filter()或其他方法）来查找第二个数组中不存在的元素。
What I tried : 我试过了
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<p id="res"></p>
<script>
var result = document.getElementById("res");
var arr = [1,2,3,1,2,3,4];
var scor = [2,3];
x = arr.map(function(item){
return scor.forEach(function(item1){
if(item1 != item)
return item;
})
})
result.innerHTML = x;
</script>
</body>
</html>
The answer should be [1,1,4] but I am getting [,,,,,,,] . 答案应该是[1,1,4]但我正在得到[,,,,,,,] 。
Where is my mistake? 我的错误在哪里？
2 个解决方案
解决方案1
1 2016-06-25 13:30:04
Your outer function (the one you pass to .map ) returns nothing, so you end up with an array full of undefined s. 您的外部函数（传递给.map函数）不返回任何内容，因此最终得到一个充满undefined s的数组。
It's equivalent to 相当于
x = [1,2,3,1,2,3,4].map(function (item) {});
解决方案2
1 已采纳 2016-06-25 13:32:38
You should not use map for this, it creates entry for every element, that's why you get bunch of undefined . 您不应该为此使用map，它会为每个元素创建一个条目，这就是为什么您会得到一堆undefined的原因。
Array.prototype.reduce would be more convenient here: Array.prototype.reduce在这里会更加方便：
var arr = [1, 2, 3, 1, 2, 3, 4]; var scor = [2, 3]; var x = arr.reduce(function(prev, curr) { if (scor.indexOf(curr) === -1) { prev.push(curr); } return prev; }, []); console.log(x);
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