JavaScript循环和范围

JavaScript的一项功能可能会引起开发人员的麻烦,与循环和作用域有关。了解有关var的循环和作用域的一些技巧,然后让

有一个特点JavaScript这可能会给开发人员带来一些麻烦,与循环和作用域有关。

举个例子:

const operations = []

for (var i = 0; i < 5; i++) { operations.push(() => { console.log(i) }) }

for (const operation of operations) { operation() }

它基本上进行迭代,并向数组中添加一个函数5次,称为操作。该功能控制台记录循环索引变量i

稍后它将运行这些功能。

这里的预期结果应该是:

0
1
2
3
4

but actually what happens is this:

5
5
5
5
5

Why is this the case? Because of the use of var.

Since var declarations are hoisted, the above code equals to

var i;
const operations = []

for (i = 0; i < 5; i++) { operations.push(() => { console.log(i) }) }

for (const operation of operations) { operation() }

so, in the for-of loop, i is still visible, it’s equal to 5 and every reference to i in the function is going to use this value.

So how should we do to make things work as we want?

The simplest solution is to use let declarations. Introduced in ES6, they are a great help in avoiding some of the weird things about var declarations.

Changing var to let in the loop variable is going to work fine:

const operations = []

for (let i = 0; i < 5; i++) { operations.push(() => { console.log(i) }) }

for (const operation of operations) { operation() }

Here’s the output:

0
1
2
3
4

How is this possible? This works because on every loop iteration i is created as a new variable each time, and every function added to the operations array gets its own copy of i.

Keep in mind you cannot use const in this case, because there would be an error as for tries to assign a new value in the second iteration.

Another way to solve this problem was very common in pre-ES6 code, and it is called Immediately Invoked Function Expression (IIFE).

In this case you can wrap the entire function and bind i to it. Since in this way you’re creating a function that immediately executes, you return a new function from it, so we can execute it later:

const operations = []

for (var i = 0; i < 5; i++) { operations.push(((j) => { return () => console.log(j) })(i)) }

for (const operation of operations) { operation() }


More js tutorials: