CODEWITHSUNDEEP
javascriptfor-loopecmascript-6
gubawhips
gubawhips

Reputation: 65

When using let in a for loop initialization, why isn't the let variable avaiable in the enclosing scope?

This has always been a lingering question for me: If block scopes are created when a let or const identifier is enclosed within curly brackets, then how is the let identifier in the initialization statement of a for loop not available in the enclosing scope but is instead available inside the curly brackets of the for loop?

(function() {
  for (let i = 0; i < 5; i++) {
   console.log(i) // logs current value of i
  }
  console.log(i) // referenceError
})()

Upvotes: 4

Views: 137

Answers (3)

J-Cake
J-Cake

Reputation: 1618

If you don't want to change your syntax, you could use var.

(function() {
    for (var i = 0; i < 5; i++) {
        console.log(i);
    }
    console.log(i); 
    // this should work.
})()

Otherwise, a variable declared with let or const has what's called Block Scope meaning that it's only visible inside the curly braces it was defined in. That's essentially the fundamental difference between let and var. A variable declared with var has Function Scope (It's visible throughout the function, even before it's lexical definition, it's value will just be undefined) 

(function() {
    console.log(i); // undefined
    var i = 10;
    console.log(i); // 10
 })();

But the example above will only work if there is no "use strict" anywhere.

Upvotes: 0

CertainPerformance
CertainPerformance

Reputation: 370759

That's just how things work. A variable declared at the top of a for loop like that is only visible inside the current iteration's for block. You can think of it a bit like this:

<loop> {
  let i = getCount();
  console.log(i) // logs current value of i
}

where getCount runs the logic that increments i.

Variables declared with let are block-scoped - it wouldn't make sense for i to be referenceable outside. If the i was visible outside of the for, what would you expect its value to be? You already have a separate i binding for every iteration of the loop. It wouldn't make sense to somewhat-arbitrarily pick one of those bindings to be visible outside.

Upvotes: 2

Jack Bashford
Jack Bashford

Reputation: 44107

Because it's a local variable in the for loop and not outside it. If you had declared it outside the loop, you could access it correctly.

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

Upvotes: 0

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