Add no-let-undefined rule

[andy-hanson]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

What changes did you make?

Added the no-let-undefined rule, which forbids let x = undefined;, preferring let x;.

[nchen63]

hasFix: true

[nchen63]

add arg initializer test?
add const x = undefined; which shouldn't fail lint

[nchen63]

lgtm, thanks!

[ajafff]

IMO this rule is a bit too specialized.
What about var? Why not check parameters and destructuring? Then it could also be renamed to no-unnecessary-initializer

[andy-hanson]

Yeah, this could work for var.

It can't be extended to parameters, though: function f(x: string | undefined = undefined) will allow f(), but function f(x: string | undefined) will not. (Besides, one should use x?: string anyway.)

For destructuring you mean to warn on this?

declare function f(): { x?: number };
const { x = undefined } = f(); // Could be `const { x } = f()`

[ajafff]

Didn't know there is this difference for parameters. So the checking would be limited to optional parameters only ...

Yes, that's what I had in mind for destructuring. All of the following examples should result in a failure:

let {foo = undefined} = someObject; // doesn't need initializer, always defaults to undefined
let [foo = undefined] = someArray; // same as above

let {foo = undefined} = {}; // is valid only if there is an initializer, but this is either by mistake or really bad practice to declare variables
let [foo = undefined] = []; // same as above