Pop Quiz!
Which of these is an infinite loop?
And guess many times console.log will be printed.
A: let
5x3 Loop
for(let i = 0; i < 5; i++){
for(let i = 0; i < 3; i++){
console.log(1)
}
}
B: var
5x3 Loop
for(var i = 0; i < 5; i++){
for(var i = 0; i < 3; i++){
console.log(1)
}
}
C: var
5x5 Loop
for(var i = 0; i < 5; i++){
for(var i = 0; i < 5; i++){
console.log(1)
}
}
D: let
5x5 Loop
for(let i = 0; i < 5; i++){
for(let i = 0; i < 5; i++){
console.log(1)
}
}
Answers
Which of these is an infinite loop?
B: var
5x3
Guess many times console.log will be printed.
A:
let
5x3 - 15 timesB:
var
5x3 - Infinite timesC:
var
5x5 - 5 timesD:
let
5x5 - 25 times
Did any of the answers surprise you? I was!
🧐 What, but why? The difference between let
and var
.
< flashback >
A junior dev came up to me for help with a bug in his JavaScript code that's causing a memory error, my intuition told me that there was an infinite loop somewhere.
One of the nested loops stood out as a red flag to me:
for(let i = 0; i < 5; i++){
for(let i = 0; i < 5; i++){
// some code
}
}
I said - Hey, you're using the same variable name for the outer and inner loop, this is going to cause an infinite loop.
JD: Huh? How come?
Me: Because the inner loop is resetting i
to 0, causing the outer loop to never exit. Come let's try this in the console, it's faster to just see it.
To my surprise, there wasn't an infinite loop, we got this:
console.log
was printed only 25 times.
Me: Hm...? That's odd. (Then I realized that the code uses let
instead of var
.) Maybe... the infinite loop only happens if you use var
it instead of let
.
We changed let
to var
, and ran it again:
console.log
was printed only 5 times.
Us: Wait whuut? Why it is printing only 5 times?
Me: ... Ahhh, that's because since this example uses var
instead of let
. let
allows you to safely declare scoped variables with the same name. But when you use var
to declare variables with the same name in different parent-child scopes, they all reference the same thing. So here, when you use var
, the inner loop is sharing the same i
as the outer loop. Meaning, when the inner loop counts i
up to 5 and exits, the outer loop exits immediately because its i
counter is set to 5 by the inner loop.
JD: Ohh... okay, then what if we set the inner loop to exit when i
is 3? I guess that produces an infinite loop?
Me: Let's find out.
console.log
was printed way beyond 15 times, and crashed the browser.
Us: 🙌 We got an infinite loop!
And what if we change var
to let
now?
"]]],[1,"p",[[0,[],0,"Ah thinking back, we have it good now with ES6."]]]]}'>
console.log
was printed only 15 times.
JD: Ok, cool. But what's the use case of var
then?
Me: Ah that's a good question. 🤔 ...none actually. There isn't a good use case for var
now that ES6 introduced let
. var
is just how we used to declare variables - but then there's this problem with variables leaking out of their scope - hence let
was proposed. Don't ask me why they decided to name the keyword let
. And this is why our eslint is configured to enforce let
over var
. :)
JD: Oh! Ok... so if this loop isn't causing the memory error, so what's causing it? (That turned out to be something else altogether.)
< /flashback >
Ah thinking back, we have it good now with ES6.