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JavaScript

Posted by Shane Harper
Shane Harper
8,864 Points

Why does "let i = 0" work in the for loop instead of "var i = 0"?

When I tried to make the for loop for the mouseover/mouseout event listeners, it did not work when i said "var i = 0." After changing it to "let i = 0", it worked. Why is this the case?

for (let i = 0; i < listItems.length; i++) { listItems[i].addEventListener('mouseover', () => { listItems[i].textContent = listItems[i].textContent.toUpperCase(); }); listItems[i].addEventListener('mouseout', () => { listItems[i].textContent = listItems[i].textContent.toLowerCase(); }); }

1 Answer

Steven Parker
Steven Parker
243,095 Points
Steven Parker
Steven Parker
243,095 Points

It's a matter of scope. Variables created with "let" have block scope, so each event listener has a separate value for "i", which is what it needs to work correctly. On the other hand, "var" has function scope, which means that all the event handlers share the same value of "i", which ends up as the value that stopped the loop; and that value is not valid for any of the listeners.

Shane Harper
Shane Harper
8,864 Points

That makes a lot more sense now - so is it generally better practice to use the "let" keyword in for loops instead of "var"?

Steven Parker
Steven Parker
243,095 Points
Steven Parker
Steven Parker
243,095 Points

The difference won't always be as significant as it is here. But it is considered good practice to use "const" where possible, and "let" otherwise.

I have to admit that I still use "var" occasionally also, but mostly as a way to make global declarations obvious. But always be mindful of the scope in any case.