In ES6, the hoisting of block-scoped variables into the for loop header in

Question

In ES6, the hoisting of block-scoped variables into the for loop header in

Do block scope variables in a for loop get hoisted above the loop header?

    var y = 4;

    for(let j = 3; j < y; j++) {
        let y = 2;
        ...
    }

Will this trigger a dead-zone error on y each time j is compared to y in the loop header? I know j is pushed down in the block scope, but what about y?

javascript ecmascript-6 hoisting

Answer 1

Answer №1

The specific section in the language specification can be found at 13.7.4

When a for statement includes a let or const declaration, it results in the creation of a new scope.

A scope is created for each iteration if the for statement contains a let declaration.

If the body of the for loop is enclosed in curly braces, then a scope is also created.

Here are some examples that demonstrate the creation of scopes:

// No additional scope
for(i = 0; i<3; i++) console.log(i);

// No additional scope
for(var i = 0; i<3; i++) console.log(i);

// Scope for 'i' and iteration scope
for(let i = 0; i<3; i++) console.log(i);


// Scope for 'i', iteration scope, and block scope
for(let i = 0; i<3; i++) {
  console.log(i);
}

Why are iteration scopes necessary? They serve the purpose of closures:

for(let i = 0; i<3; i++) {
  setTimeout(() => console.log(i), 10);
}

Output without iteration scope: 3,3,3. With iteration scope: 0,1,2

Answer 2

The specific section in the language specification can be found at 13.7.4

When a for statement includes a let or const declaration, it results in the creation of a new scope.

A scope is created for each iteration if the for statement contains a let declaration.

If the body of the for loop is enclosed in curly braces, then a scope is also created.

Here are some examples that demonstrate the creation of scopes:

// No additional scope
for(i = 0; i<3; i++) console.log(i);

// No additional scope
for(var i = 0; i<3; i++) console.log(i);

// Scope for 'i' and iteration scope
for(let i = 0; i<3; i++) console.log(i);


// Scope for 'i', iteration scope, and block scope
for(let i = 0; i<3; i++) {
  console.log(i);
}

Why are iteration scopes necessary? They serve the purpose of closures:

for(let i = 0; i<3; i++) {
  setTimeout(() => console.log(i), 10);
}

Output without iteration scope: 3,3,3. With iteration scope: 0,1,2

Answer 3

Answer №2

https://i.sstatic.net/wQE7m.jpg

My interpretation suggests that the variable 'x' is restricted to that specific scope.

Answer 4

https://i.sstatic.net/wQE7m.jpg

My interpretation suggests that the variable 'x' is restricted to that specific scope.

Answer 5

Answer №3

Are all variables declared within the block scope of a for loop hoisted above the loop header itself?

No, they are only hoisted within the block scope.

If x is compared to i in the loop header, should it result in a dead-zone error every time? I understand that i is moved down in the block scope, but why not x?

No, because when x is referenced in the loop header, it actually refers to the previously defined x set as 4 with var x = 4

You cannot access a variable declared with let before its declaration due to being in the Temporal Dead Zone (TDZ).

For example:

console.log(x) // Reference Error, x is in TDZ
let x = 4
console.log(x) // --> 4

Answer 6

Are all variables declared within the block scope of a for loop hoisted above the loop header itself?

No, they are only hoisted within the block scope.

If x is compared to i in the loop header, should it result in a dead-zone error every time? I understand that i is moved down in the block scope, but why not x?

No, because when x is referenced in the loop header, it actually refers to the previously defined x set as 4 with var x = 4

You cannot access a variable declared with let before its declaration due to being in the Temporal Dead Zone (TDZ).

For example:

console.log(x) // Reference Error, x is in TDZ
let x = 4
console.log(x) // --> 4

Answer 7

Answer №4

My initial confusion stemmed from the belief that there would only be a single block scope. I hesitated when faced with having to determine which block scope to search for the two arbitrary variables in the expression i < x, as this was an impossible task.

The proper solution involves creating two nested block scopes: one for the loop header and another for the loop body. This way, both let i and let x are visible in the loop header and only let x is visible in the loop body. Simple and straightforward.

Answer 8

My initial confusion stemmed from the belief that there would only be a single block scope. I hesitated when faced with having to determine which block scope to search for the two arbitrary variables in the expression i < x, as this was an impossible task.

The proper solution involves creating two nested block scopes: one for the loop header and another for the loop body. This way, both let i and let x are visible in the loop header and only let x is visible in the loop body. Simple and straightforward.

In ES6, the hoisting of block-scoped variables into the for loop header in

Answer №1

Answer №2

Answer №3

Answer №4

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