markdown JS：在循环中使用闭包

Posted 2021-05-05

## Using a closure within a loop

Closures are sometimes brought up in an interview so that the interviewer can gauge how familiar you are with the language, and whether you know when to implement a closure.

A closure is basically when an inner function has access to variables outside of its scope. Closures can be used for things like implementing privacy and creating function factories. A common interview question regarding the use of closures is something like this:

Write a function that will loop through a list of integers and print the index of each element after a 3 second delay.

A common (incorrect) implementation I’ve seen for this problem looks something like this:
```sh
const arr = [10, 12, 15, 21];

for (var i = 0; i < arr.length; i++) {
  setTimeout(function() {
    console.log('The index of this number is: ' + i);
  }, 3000);
}
```

If you run this you’ll see that you actually get the 4 printed out every time instead of the expected 0, 1, 2, 3 after a 3 second delay.

To correctly identify why this is happening it would be useful to have an understanding of why this happens in JavaScript, which is exactly what the interviewer is trying to test.

The reason for this is because the setTimeout function creates a function (the closure) that has access to its outer scope, which is the loop that contains the index i. After 3 seconds go by, the function is executed and it prints out the value of i, which at the end of the loop is at 4 because it cycles through 0, 1, 2, 3, 4 and the loop finally stops at 4.

There are actually a few ways of correctly writing the function for this problem. Here are two of them:

```sh
const arr = [10, 12, 15, 21];

for (var i = 0; i < arr.length; i++) {
  // pass in the variable i so that each function 
  // has access to the correct index
  setTimeout(function(i_local) {
    return function() {
      console.log('The index of this number is: ' + i_local);
    }
  }(i), 3000);
}
```

```sh
const arr = [10, 12, 15, 21];
for (let i = 0; i < arr.length; i++) {
  // using the ES6 let syntax, it creates a new binding
  // every single time the function is called
  // read more here: http://exploringjs.com/es6/ch_variables.html#sec_let-const-loop-heads
  setTimeout(function() {
    console.log('The index of this number is: ' + i);
  }, 3000);
}
```