Variant of no-unused-expressions that allows type assertions?

[mkscrg]

It's sometimes helpful to write a bare type assertion with no side-effects. For example when discriminating a disjoint union in a switch:

type Action
  = { type: 'FOO', doFoo(_: number): void }
  | { type: 'BAR', doBar(_: string): void };

function handleAction(act: Action): void {
  switch (act.type) {
    case 'FOO':
      doFoo(10);
      break;
    case 'BAR':
      doBar('hello');
      break;
    default:
      (act: empty); // type error unless we've exhausted the possibilities for `act.type`
  }
}

Unfortunately these are caught by ESLint's no-unused-expressions rule.

Is there a clean way to "fix" this with eslint-plugin-flowtype? Maybe we could expose a flowtype/no-unused-expressions that additionally ignores type assertions?

[mkscrg]

Scratched this itch with #272

[bradennapier]

Love this one! Thanks!

[cinic]

Not working for me.

Eslint: 4.14
Eslint-plugin-flowtype: 2.44.0
babel-eslint: 8.2.1

Eslint error: Expected an assignment or function call and instead saw an expression. (no-unused-expressions).

const reducer = (state: ConfigState = initialState, action: ConfigActions): ConfigState => {
  switch (action.type) {
    case APP_CONFIG_SUCCESS:
      return {
        ...state,
        config: action.config,
      };
    default:
      (action: empty); // eslint caught error
      return state;
  }
};

[pronebird]

Hey, this doesn't work for me either, my eslintrc:

{
  "extends": [
    "eslint:recommended",
    "plugin:react/recommended",
    "plugin:promise/recommended",
    "plugin:flowtype/recommended",
    "prettier"
  ],
  "plugins": ["react", "flowtype", "promise"],
  "rules": {
    "no-unused-expressions": "error",
  }
}

How do you guys disable no-unused-expressions because ESLint picks up the ones that belong to Flow?