Let variable declarations

JavaScript
1

I’ve been spending some time with Mozilla docs lately and noticed some lines that were like

let whatever = 123;

instead of the expected

var whatever = 123;

“let” being a common word it took a while to find info on it, but as best I could find it’s for “block level scope”.

Am I missing something? Is this something useful or just a way to avoid scope errors?

2

It’s only relevant where JavaScript 1.7 is supported.

Yes, let is to define a block-scope variable. You can read more about it at https://developer.mozilla.org/en/JavaScript/Reference/Statements/let

Any code though that might be run on Internet Explorer must remain compatible with JScript, which is Microsoft’s own variation of JavaScript 1.3 instead.

3

Thanks for the link to the docs Paul, searching for “let” wasn’t getting me anywhere. (“1.7” would have been better if I had only known).

I can see where it would come in handy. Right now if a function has a lot of for loops I make sure the “index” variables are all named different. Using let would make things easier and less error prone.

4

I find that using MDN as the keyword (for Mozilla Developer Network) helps Google get to the right place. That way a successful result can be obtained by searching for:
mdn let

I can see where it would come in handy. Right now if a function has a lot of for loops I make sure the “index” variables are all named different. Using let would make things easier and less error prone.[/QUOTE]

What helps is to place declared variables right at the start.

For example, with 3 loops:


var i;

for (i = 0; i < foo.length; i += 1) {
    ...
}
for (i = 0; i < bar.length; i += 1) {
    ...
}
for (i = 0; i < baz.length; i += 1) {
    ...
}

Notice how the initial expression for each for loop doesn’t use var declarations? This helps to make them more portable and consistent.

Or when they are nested:


var x, y, z;

for (x = 0; x < foo.length; x += 1) {
    for (y = x; y < bar.length; y += 2) {
        for (z = y; z < baz.length; z += 1) {
            ...
        }
        ...
    }
    ...
}

Mind you, with that last, lot I’d be tempted to break them up in to separate functions, and use the forEach method on them instead.