Unused underscore variables not being ignored by lint rules

[chmeyers]

TypeScript Version: 2.7.0-dev.20171031, 2.6.1 (Regression from 2.5.3)

Expected behavior:
Variables with names that begin with an underscore do not cause an error if not used.

Actual behavior:
components/panel/panel.ts(22,11): error TS6133: '_someVariable' is declared but its value is never read.

Underscores may be used to indicate unused parameters. A local variable such as const _ = 0; will still be flagged. Could you provide a complete example to show a regression?

[chmeyers]

Originally noticed this on this variable:
https://github.com/yebrahim/datalab/blob/52ab35257d8c338ba10dfbf631fa756e603e37c7/sources/web/datalab/polymer/components/auth-panel/auth-panel.ts#L22

Let me see if I can make a self-contained repro.

this._signedIn is written to but it doesn't appear to be read anywhere. This is newly detected in ts2.6 (#17752).

[chmeyers]

Repro:
class TestClass {
private _shouldBeIgnored: boolean;

_someFunction() {
this._shouldBeIgnored = true;
}
}

tsc --noUnusedLocals test.ts
(Works in 2.5.3, fails on 2.6.1 (and latest nightly))

[chmeyers]

In our particular case these variables are read by the html page.

We don't support underscore ignores for private fields, only for parameters. Maybe you could use // @ts-ignore (#19139) or a dummy read?
Alternately, if the field is being accessed from the outside you might reconsider marking it private...

[nikklassen]

My use case for this is array destructing. I have a function that returns a "tuple" (an array like [int, string]) and in some cases I would like to ignore one of the elements i.e. const [_foo, bar] = func() but keep the tuple-like semantics. const bar = func()[1] no longer communicates that this is a tuple.

[aluanhaddad]

Nik Klassen (@nikklassen) you can use a leading , as in

const [ , bar ] = func();

[petemcwilliams]

This warning

is written to but it doesn't appear to be read anywhere

Would be more useful than the one I was getting in version 2.6.1

is declared but its value is never read.

Because it is a new check I was searching for quite a while to figure this out, seeing the variable was being used (written to) in the ts.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.