javascript 在 forEach 循环中访问它会导致未定义
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Accessing this in a forEach loop results in undefined
提问by Mike Rifgin
I'm iterating through an array using forEach in one of my Class's methods. I need access to the instance of the class inside the forEach but thisis undefined.
我在我的类的方法之一中使用 forEach 遍历数组。我需要访问在foreach内部的类的实例，但是这是不明确的。
var aGlobalVar = {};
(function () {
"use strict";
aGlobalVar.thing = function() {
this.value = "thing";
}
aGlobalVar.thing.prototype.amethod = function() {
data.forEach(function(d) {
console.log(d);
console.log(this.value);
});
}
})();
var rr = new aGlobalVar.thing();
rr.amethod();
I have a fiddle I'm working on here: http://jsfiddle.net/NhdDS/1/.
我有一个我在这里工作的小提琴：http: //jsfiddle.net/NhdDS/1/。
回答by T.J. Crowder
In strict mode if you call a function notthrough a property reference and without specifying what thisshould be, it's undefined.
在严格模式下，如果你不通过属性引用调用一个函数，并且没有指定this应该是什么，它是undefined.
forEach(spec| MDN) allows you to say what thisshould be, it's the (optional) second argument you pass it:
forEach( spec| MDN) 允许你说this应该是什么，这是你传递的（可选）第二个参数：
aGlobalVar.thing.prototype.amethod = function() {
data.forEach(function(d) {
console.log(d);
console.log(this.value);
}, this);
// ^^^^
}
Alternately, arrow functionswere added to JavaScript in 2015. Since arrows close over this, we could use one for this:
或者，箭头函数在 2015 年被添加到 JavaScript 中。由于箭头关闭this，我们可以使用一个：
aGlobalVar.thing.prototype.amethod = function() {
data.forEach(d => {
console.log(d);
console.log(this.value);
});
}
回答by Qantas 94 Heavy
Since you're using strict mode, when a function is called that isn't a property of an object, thiswill have the value undefinedby default (not the global object). You should store its value manually:
由于您使用的是严格模式，因此当调用不是对象属性的函数时，默认情况下this将具有该值undefined（不是全局对象）。您应该手动存储其值：
var aGlobalVar = {};
(function () {
"use strict";
aGlobalVar.thing = function () {
this.value = "thing";
};
aGlobalVar.thing.prototype.amethod = function () {
var self = this;
data.forEach(function (element) {
console.log(element);
console.log(self.value);
});
};
})();
var rr = new aGlobalVar.thing();
rr.amethod();
Nowadays, with ES2015 you can also use arrow functions, which uses the thisvalue of the outside function:
如今，在 ES2015 中，您还可以使用箭头函数，它使用this外部函数的值：
function foo() {
let bar = (a, b) => {
return this;
};
return bar();
}
foo.call(Math); // Math
T.J. Crowder's solution of using the second argument of forEachalso works nicely if you don't like the idea of the temporary variable (ES5 code: works in pretty much any browser these days, except IE8-).
forEach如果您不喜欢临时变量的想法，TJ Crowder 使用的第二个参数的解决方案也能很好地工作（ES5 代码：现在几乎可以在任何浏览器中使用，除了 IE8-）。
回答by Narcis
What I had to to is add thisin the every forEach that I was using (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach). Binding in the constructor is not needed, as I am using arrow functions. So now my code is:
我必须this在我使用的每个 forEach 中添加（参见https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach）。不需要在构造函数中绑定，因为我使用的是箭头函数。所以现在我的代码是：
resetPressed = () => {
this.transport_options.forEach(function (transport_option) {
this.pressed_percentages.forEach(function (percentage) {
filters[transport_option][percentage] = false;
}, this)
}, this);
filters.isFilterActive = false;
this.setState({
filtersState: filters,
opacity: filters.isFilterActive ? 1 : 0.5
});
}
<TouchableHighlight
underlayColor={'transparent'}
onPress={this.resetPressed}
style={styles.iconView}>
