no-unused-vars deprecated, noUnusedLocals ignored

[joeytwiddle]

Steps to reproduce

nvm use v10
npm install -g @loopback/cli
lb4 lb4-lint-test -y
cd lb-4-lint-test   # sic
# Add an unused variable
echo "const foo = 'hello';" >> src/index.ts
npm run test

Current Behavior

Output:

no-unused-variable is deprecated. Since TypeScript 2.9. Please use the built-in compiler checks instead.

ERROR: .../lb-4-lint-test/src/index.ts[17, 7]: 'foo' is declared but its value is never read.

Expected Behavior

Tests pass without error.

Towards a solution

On the web I read that, with no-unused-vars now deprecated in tslint, we are supposed to add noUnusedLocals to tsconfig.json instead:

  "compilerOptions": { "noUnusedLocals": false },

But that doesn't work either!

Quick fix

UPDATE February 2019: This workaround is no longer helping. The only way I now know to allow unused variables to pass linting, is to not upgrade typescript and tslint!

What does did work, was if I changed the tsconfig.json as above, and I also changed how linting is called in our package.json:

    "tslint": "tslint -p tsconfig.json",

^^ Hopefully this can help anyone who is stuck with this.

But notably when I try to do closer to what lb-tslint does:

    "tslint": "tslint -p tsconfig.json -c tslint.build.json",

then the error comes back again!

So I'm guessing this is actually a bug with tslint? Anyway I will leave this issue here for anyone who is having trouble right now.

[raymondfeng]

See palantir/tslint#4100

[raymondfeng]

BTW, noUnusedLocals is only available as a command line option for TypeScript. It cannot be set in tsconfig.json.

[JoshuaKGoldberg]

cannot be set in tsconfig.json

You can set it as a child of "compilerOptions":

{
  "compilerOptions": {
    "module": "esnext",
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "strict": true,
    "target": "es2017"
  }
}

[raymondfeng]

@JoshuaKGoldberg Thanks for pointing out. If I remember correctly, we cannot override such rules in TypeScript like // tslint:disable:....

[joeytwiddle]

To restate the problem:

• tslint is ignoring the setting noUnusedLocals: false in tsconfig.json
  It is still complaining about unused locals.

I finally found the solution. In tslint.json, add this rule:

"rules": {
  "no-unused": false
}

The change was documented here: 78c9d36 but I only find that now. :-)