ES6: Compare Scopes of the var and let Keywords

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.

Fix the code so that i declared in the if statement is a separate variable than i declared in the first line of the function. Be certain not to use the var keyword anywhere in your code.

This exercise is designed to illustrate the difference between how var and let keywords assign scope to the declared variable. When programming a function similar to the one used in this exercise, it is often better to use different variable names to avoid confusion.