Question

如何访问foreach中的范围以获取变量值？

var fruitCollection = ['Grape','Orange','Banana'];

// give me a .txt file with the recipe for each fruit
var fruitsNeeded = function(){
    fruitCollection.forEach(function(fruit) {
        var fruitRecipe  = fruit.toLowerCase() + '.txt';

        return fruitRecipe;
    });
}

fruitsNeeded(); // undefined

编辑预期输出：

// grape.txt
// orange.txt
// banana.txt

Answer 1

您无法从ECHO ON函数

返回值

forEach（）为每个数组元素执行一次回调函数;与map（）或reduce（）不同，始终返回值未定义且不可链接。

您可以使用forEach功能

map

ES6摘录：

＆＃13;

var fruitCollection = ['Grape','Orange','Banana'];

var fruitsNeeded = function(){
    return fruitCollection.map(function(fruit) {
         return fruit.toLowerCase() + '.txt';
    });
}

＆＃13;

Answer 2

另一个选择是构建一个数组并返回整个数组：

var fruitCollection = ['Grape','Orange','Banana'];

// give me a .txt file with the recipe for each fruit
var fruitsNeeded = function(){
    var fruitRecipeList = [];
    fruitCollection.forEach(function(fruit) {
        var fruitRecipe  = fruit.toLowerCase() + '.txt';
        fruitRecipeList.push(fruitRecipe);
    });
    return fruitRecipeList;
}

fruitsNeeded();

Answer 3

forEach没有返回值;它只是一种在数组上执行的方法。您可能需要map：

var fruits = fruitCollection.map(function(fruit) {
  var fruitRecipe = fruit.toLowerCase() + '.txt';
  return fruitRecipe;
});
// fruits = ['grape.txt','orange.txt','banana.txt']

Answer 4

而不是从forEach返回一个值，您可以将它们添加到另一个全局范围变量中。
像这样：

var fruitCollection = ['Grape','Orange','Banana'];
var fruitFiles = [] ;

// give me a .txt file with the recipe for each fruit
var fruitsNeeded = function(){
    fruitCollection.forEach(function(fruit) {
        var fruitRecipe  = fruit.toLowerCase() + '.txt';

        fruitFiles.push(fruitRecipe) ;
    });
return fruitFiles ;
}

fruitsNeeded(); // ["grape.txt", "orange.txt", "banana.txt"]

我希望这有帮助，左雅各布

返回变量.forEach（）

4 个答案: