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I understand the block vs. function scoping of let & var (or I thought I did). Recently I ran across some JavaScript that used let in function scope where I thought it would be equivalent to using var, like:

function xyz ( abc ) {
    let mno = abc - 1;   /* or whatever */
    ...
}

Is there any functional difference between that and using var there? Or is it just a stylistic thing?

John Hascall
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  • The answer you are looking for: http://stackoverflow.com/questions/762011/let-keyword-vs-var-keyword – Inacio Schweller Jan 25 '16 at 02:28
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    Some people believe `var` should never have been in the language, and would use `let` in all cases, as promoting good practices. – Amadan Jan 25 '16 at 02:29

3 Answers3

4

One difference inside function scope would be that temporal dead zone can occur if you try to reference a let before it's defined:

function example() {
    console.log(a); // undefined
    console.log(b); // ReferenceError: b is not defined
    var a = 1;
    let b = 2;
}
dsgriffin
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  • @max Not sure, but I remember reading a while back that he was basically rushed by Netscape to come back to them in 10 days with a new language prototype (hence all the strange js bugs that carry through to today) – dsgriffin Jan 25 '16 at 02:49
3

No difference whatsoever. It's either a stylistic thing or a misunderstanding of how var and let work. Perhaps the original author (the person who wrote the function you saw) thought that let is the same as const (which is almost the case in languages like Swift). In other words, sometimes people bring styles/syntax from one language into another because the declarations look similar, but in reality mean completely different things.

I understand the block vs. function scoping of let & var (or I thought I did)

Don't worry, you do. let is for block-scoping, just as you pointed out.

Community
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Josh Beam
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  • Thanks, I was burned by perl's ridiculous `my` scoping once, so I wanted to make sure there wasn't some silly corner case I was unaware of. – John Hascall Jan 25 '16 at 02:32
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    No problem. No weird corner cases here, as far as I'm aware. – Josh Beam Jan 25 '16 at 02:33
  • Except that `var` hoists while `let` works well - in a less silly and confusing manner. Many have advocated that you should replace `var` everywhere because its just less confusing for programmers from other languages. – max Jan 25 '16 at 02:38
0

According to this answer, there is no difference in the case you described, they're functionally the same as they're both in the scope of the function. As you guessed, probably just the person (or linter) wanting to be consistent in defining variables.

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Clavin
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