Циклы и область действия JavaScript

Есть одна особенность JavaScript, которая может вызвать у разработчиков несколько головных болей, связанных с циклами и областью видимости. Изучите некоторые трюки с циклами и областью видимости с помощью var и позвольте

Есть одна особенностьJavaScriptэто может вызвать у разработчиков несколько головных болей, связанных с циклами и областью видимости.

Возьмем этот пример:

const operations = []

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

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

Он в основном выполняет итерацию и 5 раз добавляет функцию в массив, называемый операциями. Эта функциональная консоль регистрирует переменную индекса циклаi.

Позже он запускает эти функции.

Ожидаемый результат здесь должен быть:

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() }


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