[英]JavaScript Uncaught TypeError: Cannot convert undefined or null to object
I have a question,我有个问题,
// case 1
const stack = [];
[1,2,3].forEach(stack.push);
This will throw Error这将抛出错误
Uncaught TypeError: Cannot convert undefined or null to object
at push (<anonymous>)
at Array.forEach (<anonymous>)
at <anonymous>:1:9
But this will be fine,不过这样就好了
// case 2
const stack = [];
[1,2,3].forEach(element => stack.push(element));
console.log(stack); // [1,2,3]
And if you bind stack with this
referring to itself,如果你用
this
引用自身来绑定堆栈,
// case 3
const stack = [];
[1,2,3].forEach(stack.push.bind(stack));
console.log(stack); // (9) [1, 0, Array(3), 2, 1, Array(3), 3, 2, Array(3)]
It also works in another way.它也以另一种方式工作。
How can these happen?这些怎么可能发生? What is the difference between a method(case 1) and an arrow function(case 2)?
方法(案例 1)和箭头函数(案例 2)有什么区别?
stack.push
references Array.prototype.push
. stack.push
引用Array.prototype.push
。 This:这个:
const stack = [];
[1,2,3].forEach(stack.push);
doesn't work because it's equivalent to:不起作用,因为它相当于:
const stack = [];
[1,2,3].forEach(Array.prototype.push);
or或者
const callback = Array.prototype.push;
callback(1, 0, [1, 2, 3]);
callback(2, 1, [1, 2, 3]);
callback(3, 2, [1, 2, 3]);
which doesn't work because there is no this
context of the array to push to.这不起作用,因为没有要推送到的数组的
this
上下文。
Method 2 works because you're doing => stack.push(element)
- .push
is being called with a calling context of stack
, rather than stack.push
being passed as a callback.方法 2 之所以有效,是因为您正在执行
=> stack.push(element)
- .push
是使用stack
的调用上下文调用的,而不是stack.push
作为回调传递的。 (When passed as a callback, the calling context gets lost unless you bind the function, which you do in the 3rd method). (当作为回调传递时,调用上下文会丢失,除非您在第三种方法中绑定 function)。
For another example:再举一个例子:
const obj = { fn() { console.log('fn. this is:', this); } }; const callback = obj.fn; // this does not refer to the object now: callback(); // but this will: obj.fn();
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.