Rule Proposal: `prefer-smaller-scope`

Currently prefer-const has two behaviors.

1. detecting it’s never modified.
2. detecting it can be moved to a smaller scope.

This seems to be confusing people (e.g. #5283, #5251) (I’m sorry…). So this proposal is separating the 2nd behavior to new rule: prefer-smaller-scope.

prefer-small-scope is aimed at eliminating variables in too wide scope. This rule warns variables which can move to a smaller scope. This improves readability because the scope which can read/write the variable gets smaller.

The following patterns are considered problems:

/* eslint prefer-smaller-scope: 2 */

let foo;        /*error 'foo' declaration can move into a block 'if (a) {...}'.*/
if (a) {
    foo = 100;
    bar(foo);
} else {
    bar(200);
}

The following patterns are not considered problems:

/* eslint prefer-smaller-scope: 2 */

let foo;
if (a) {
    foo = 100;
} else {
    foo = 200:
}
baz(foo);

let foo;
try {
    foo = bar();
} catch (err) {
    foo = 0:
}
baz(foo);

Then the 2nd behavior of prefer-const is removed. I think this may be not a breaking change because the rule is defused than now.

If accepted, I’m happy to work.

[mysticatea]

I’m sorry, I’m closing for now.

I have encountered a hard problem. prefer-smaller-scope needs to move variable declarations, it’s very difficult to distinguish if it can move the declarations because of references in their initializers. I will revisit this after I investigate the way of type/value tracking in our scope analysis.

[kaicataldo]

@mysticatea Where does this stand?