为什么不能在for循环中声明变量？

Question

Why can't I declare a variable inside a for loop below?

Wrong:

  for(let i = 1; i <= num; i++){
    let factorials = [];
    if(num % i === 0){
      factorials.push(i);
    }
  }
// ReferenceError: factorials is not defined

Correct:

  let factorials = [];
  for(let i = 1; i <= num; i++){
    if(num % i === 0){
      factorials.push(i);
    }
  }

Answer 1

If you define a variable inside of { } . It means that the variable is only alive inside of the { } . It's called a scope.

You have a scope using if/while/for/function ...

In the following example, the factorials variable is alive only inside of one loop iteration of the for . Which means that you are recreating a factorials variable every time you loop.

  for(let i = 1; i <= num; i++){
    const factorials = [];

    if(num % i === 0){
      factorials.push(i);
    }
  }

---

If you want to make it work, you have to create your variable outside of the for so it will not be recreate after every iteration, and keep it's values.

  const factorials = [];

  for(let i = 1; i <= num; i++){
    if(num % i === 0){
      factorials.push(i);
    }
  }

Answer 2

In the first code snippet, you are declaring factorials inside the for loop block, hence it will not be visible outside the for loop block.

It is working in the 2nd snippet because you declared it outside of block.

Answer 3

Why can't I declare a variable inside a for loop below?

No, you can.

Here is the proof.

 let num = 5; for (let i = 1; i <= num; i++) { let factorials = []; if (num % i === 0) { factorials.push(i); } } let factorials = []; for (let i = 1; i <= num; i++) { if (num % i === 0) { factorials.push(i); } } console.log("No Error.") 

Answer 4

Yes, you can, but if you declare the factorials-array in the loop it's only available in that closure. So consoling it out of the loop won't work.

Here's a very informative article about closures: https://javascript.info/closure

The whole javascript.info-site is very interesting, I am studying it for now to strengthen my javascript basics and fill not known informations-holes, although I am into web developing since years. But it never hurts to get more knowledge, even if its the basics.

Also I would advice you to use "use strict;" from beginning when developing, it helps to keep your scripts more clean, disallowing sources of errors.