Globally and Locally Scopes

JavaScript
1

Could anyone tell me why this code get 3 ?
loop 0, 1, 2 but how come 3 ??


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

Link to the challenge:
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/es6/compare-scopes-of-the-var-and-let-keywords

2

The i variable takes 3 as its last value, because it’s the value that fails the for-loop condition.

1 Like
3

I don’t understand Why does i takes 3 as its last value ? can you explain a little bit more

4

i starts at 0. Then it checks if i < 3, which is true, so it proceeds to run the loop once. After all that, i++ runs, so i is now 1.

Then it checks again if i < 3. Again it’s true, and runs the loop again. Then i is incremented to 2.

Then it checks again if i < 3. Again it’s true, and runs the loop again. Then i is incremented to 3.

Then it checks again if i < 3. Now this is false, because 3 is not less than itself. The for-loop stops, then the console.log is ran. The function uses that last value of i, and you get 3.

5

I understand the concept of loop (it loop until the condition no longer true) please check the code below for me

var printNumTwo;
for (var i = 0; i < 3; i++) {
// console.log(i) get 0, 1, 2   exactly what i expected 
  if(i === 2){   // but here the if statement what does it mean ?, (if i === 2) then run printNumTwo function which return i ? 
    printNumTwo = function() {
      return i;
    };
  }
}
console.log(printNumTwo());
// 3

ps. I already passed this challenge and i understand how does globally and locally of variable declaration works but this code i’m asking still stuck in my mind bcoz it still doesn’t make sense for me right now

6

It’s true that printNumTwo gets assigned that function when i is 2. But it doesn’t mean that the printTwoNum function will return 2. It will return whatever the value of i is when you call it. Before you called printNumTwo in the last line, the i variable takes the value of 3, as per for-loop mechanics (try console.log(i) immediately after the for-loop). So that’s what the function returns.

1 Like
7

Perhaps to supplement, the code inside the function is executed after function invocation not after declaration. You can declare a function but if you don’t invoke it, it won’t execute. The if condition in the for loop evaluates to true when i = 2 and then assigns the function expression to the global variable PrintNumTwo
but doesn’t execute. The if loop continues until i is 3 then the function is invoked when the for loop is exited. Why is the i still accessible outside the for loop? It’s because its declaration is hoisted to the top of the scope. Meaning it’s a global variable as well because you have used var keyword for declaring it.