tslint how to disable error "someVariable is declared but its value is never read"

Any parameter name starting with _ is exempt from the check. Use _myVariable instead of myvariable to remove this warning.

Fist question:

Edit the file:tsconfig.json, adding/modifying key "noUnusedLocals": false.

You'll need to restart the server.

Second question:

If it is a tslint error; VS Code shows, in the error message, the rule that's been applied.

Identifier 'doc' is never reassigned; use 'const' instead of 'let'. (prefer-const)

The prefer-const rule in this case.

Add this line just before the line which causes the error:

  /* tslint:disable:no-unused-variable */

You will no longer receive the tslint error message.

This is a better solution than turning off the error for you whole codebase in tslint.conf because then it wouldn't catch variables that really aren't used.

New solution

First, turn off noUnusedLocals in tsconfig.json:

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

Then fix eslint rules in .eslintrc.js:

module.exports = {
  rules: {
    'no-unused-vars': 'off',
    '@typescript-eslint/no-unused-vars': ['error', { 'varsIgnorePattern': '^_', "argsIgnorePattern": "^_" }],
  },
};

And If using @typescript-eslint/naming-convention rule should add leadingUnderscore: 'allow', For example, if you are using Airbnb config:

'@typescript-eslint/naming-convention': [
  'error',
  {
    selector: 'variable',
    format: ['camelCase', 'PascalCase', 'UPPER_CASE'],
    leadingUnderscore: 'allow',
  },
  {
    selector: 'function',
    format: ['camelCase', 'PascalCase'],
  },
  {
    selector: 'typeLike',
    format: ['PascalCase'],
  },
],

Note: you should update all related eslint packages in package.json to the latest version manually.

Extend the tsconfig.json with dev.tsconfig.json

And run the command tsc -p ./dev.tsconfig.json

This will disable the unused variable and unused parameter in development

Inside dev.tsconfig.json:

{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "noUnusedLocals": false,
    "noUnusedParameters": false,
   }
}

I am using typescript": "2.9.1" with tslint": "^5.10.0.

I was getting tons of error such as

Property 'logger' is declared but its value is never read.

Also, I observed that I was getting a warning when running ng-lint

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

So, I removed the no-unused-variable rule fromt tslint.json - and that seems to solve the problem for me.

There are two type of variables and you can do it in two ways

argsIgnorePattern varsIgnorePattern no-unused-vars: ["error", { "argsIgnorePattern": "^_" }] no-unused-vars: ["error", { "varsIgnorePattern": "^_" }]

Both these rule in eslint will ignore any function arguments and variables that starts with _ sign respectively

I saw a solution on this web site: https://phpenthusiast.com/blog/angular-form-ngform-and-two-ways-data-binding.

it helped me but only 50% of it

This is my modified code:

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { FormsModule } from '@angular/forms';// i added this line and one more line.
import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';

import { DheerajComponent } from './dheeraj/dheeraj.component'
@NgModule({
  declarations: [
    AppComponent,
    DheerajComponent
  ],
  imports: [
    BrowserModule,
    AppRoutingModule, 
    FormsModule, // this one. remaining all default code
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { 
}

In react if you have some thing like this

export default class Body extends Component {
    
    let data = null; // somethings like this line

    // ...
    // ...

convert it to

export default class Body extends Component {
    
    data = null; // somethings like this line

    // ...
    // ...

If someone needs to disable it just for a moment, when developing, the best way may be to just run the tsc compiler with an additional command line flag:

$npx tsc -w --noUnusedLocals false --noUnusedParameters false

Another way to avoid this is to create a get-method for every variable you have, like this:

get variablename():variabletype{return this.variablename;}