typescript 'this' 在 foreach 循环中未定义
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'this' is undefined inside the foreach loop
提问by user1892775
I am writing some typescript code and iterating an array. Inside the loop, I am trying to access 'this' object to do some processing as:
我正在编写一些打字稿代码并迭代一个数组。在循环内部，我试图访问“this”对象以进行一些处理：
console.log('before iterate, this = ' +this);
myarray.days.forEach(function(obj, index) {
console.log('before transform, this : ' + this);
this.datePipe.transform...
});
but this fails, as it complains that 'this' is undefined 'this' object prints correctly as [object object] before/outside the loop, but inside the loop, it is undefined. Why is that? And what is the fix for that?
但这失败了，因为它抱怨 'this' 未定义 'this' 对象在循环之前/外部正确打印为 [object object]，但在循环内部，它是未定义的。这是为什么？对此有什么解决方法？
回答by Nitzan Tomer
You need to either use an arrow function:
您需要使用箭头函数：
myarray.days.forEach((obj, index) => {
console.log('before transform, this : ' + this);
this.datePipe.transform...
});
Or use the bind method:
或者使用绑定方法：
myarray.days.forEach(function(obj, index) {
console.log('before transform, this : ' + this);
this.datePipe.transform...
}.bind(this));
The reason is that when passing a regular function as a callback, when it is invoked the thisis not actually preserved.
The two ways which I mentioned above will make sure that the right thisscope is preserved for the future execution of the function.
原因是当将常规函数作为回调传递时，当它被调用时this实际上并没有被保留。
我上面提到的两种方法将确保this为函数的未来执行保留正确的范围。
回答by rc.adhikari
Add the thisas a parameter for callback.
添加this作为回调的参数。
Adding }, this);instead of }.bind(this));should resolved issue in Angular.
添加}, this);而不是}.bind(this));应该解决 Angular 中的问题。
Thus, should look like:
因此，应该看起来像：
myarray.days.forEach(function(obj, index) {
console.log('before transform, this : ' + this);
this.datePipe.transform...
}, this);
回答by Eray T
Try this:
尝试这个：
myarray.days.forEach( (obj) => {
console.log('before transform, this : ' + this);
this.datePipe.transform...
});
