ES6: Compare Scopes of the var and let Keywords

Posted by admin at March 3, 2020

When you declare a variable with the var keyword, it is declared globally, or locally if declared inside a function.

The let keyword behaves similarly, but with some extra features. When you declare a variable with the let keyword inside a block, statement, or expression, its scope is limited to that block, statement, or expression.

For example:

var numArray = [];
for (var i = 0; i < 3; i++) {
  numArray.push(i);
}
console.log(numArray);
// returns [0, 1, 2]
console.log(i);
// returns 3

With the var keyword, i is declared globally. So when i++ is executed, it updates the global variable. This code is similar to the following:

var numArray = [];
var i;
for (i = 0; i < 3; i++) {
  numArray.push(i);
}
console.log(numArray);
// returns [0, 1, 2]
console.log(i);
// returns 3

This behavior will cause problems if you were to create a function and store it for later use inside a for loop that uses the i variable. This is because the stored function will always refer to the value of the updated global i variable.

var printNumTwo;
for (var i = 0; i < 3; i++) {
  if (i === 2) {
    printNumTwo = function() {
      return i;
    };
  }
}
console.log(printNumTwo());
// returns 3

As you can see, printNumTwo() prints 3 and not 2. This is because the value assigned to i was updated and the printNumTwo() returns the global i and not the value i had when the function was created in the for loop. The let keyword does not follow this behavior:

'use strict';
let printNumTwo;
for (let i = 0; i < 3; i++) {
  if (i === 2) {
    printNumTwo = function() {
      return i;
    };
  }
}
console.log(printNumTwo());
// returns 2
console.log(i);
// returns "i is not defined"

i is not defined because it was not declared in the global scope. It is only declared within the for loop statement. printNumTwo() returned the correct value because three different i variables with unique values (0, 1, and 2) were created by the let keyword within the loop statement.

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