Why doesn't the let variable used in a for loop initialization extend its scope to the enclosing block?

Question

Why doesn't the let variable used in a for loop initialization extend its scope to the enclosing block?

I've always been puzzled by this question: If block scopes are generated when a let or const identifier is enclosed within curly brackets, then how come the let identifier in the initialization statement of a for loop isn't accessible in the outer scope but is only 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
})()

javascript for-loop ecmascript-6

Answer 1

Answer №1

That's just the nature of how programming works. When a variable is declared at the top of a for loop like that, it is only accessible within the current iteration's block. You can visualize it as follows:

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

In this scenario, getCount handles the logic to increment i.

Variables defined using let have block scope - it wouldn't be logical for i to be accessible outside of the for loop. If i could be accessed outside of the loop, what value would you anticipate it to hold? Each iteration of the loop has its own separate i binding. It wouldn't make sense to randomly choose one of those bindings to be accessible externally.

Answer 2

That's just the nature of how programming works. When a variable is declared at the top of a for loop like that, it is only accessible within the current iteration's block. You can visualize it as follows:

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

In this scenario, getCount handles the logic to increment i.

Variables defined using let have block scope - it wouldn't be logical for i to be accessible outside of the for loop. If i could be accessed outside of the loop, what value would you anticipate it to hold? Each iteration of the loop has its own separate i binding. It wouldn't make sense to randomly choose one of those bindings to be accessible externally.

Answer 3

Answer №2

The reason you are unable to access the variable is due to its scope being limited to the for loop. If it was declared outside of the loop, you would be able to access it properly.

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

Answer 4

The reason you are unable to access the variable is due to its scope being limited to the for loop. If it was declared outside of the loop, you would be able to access it properly.

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

Answer 5

Answer №3

If you prefer not to alter your syntax, you have the option of using var.

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

Alternatively, a variable declared with let or const possesses what is known as Block Scope, meaning it is only accessible within the curly braces where it was initialized. This represents the primary distinction between let and var. A variable declared with var has Function Scope (it is visible throughout the function, even before its lexical declaration, but its value will be undefined)

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

However, the above example will only function properly if there is no "use strict" present.

Answer 6

If you prefer not to alter your syntax, you have the option of using var.

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

Alternatively, a variable declared with let or const possesses what is known as Block Scope, meaning it is only accessible within the curly braces where it was initialized. This represents the primary distinction between let and var. A variable declared with var has Function Scope (it is visible throughout the function, even before its lexical declaration, but its value will be undefined)

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

However, the above example will only function properly if there is no "use strict" present.

Why doesn't the let variable used in a for loop initialization extend its scope to the enclosing block?

Answer №1

Answer №2

Answer №3

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