Warn/error on pointless expression?

[donaldpipowitch]

This is a feature request.

This is an example to some similar code I recently encountered. It was a little bit more complex in the real code.

interface Foo {
  bar?: {
    baz?: {
      nope?: number;
    };
  };
}

const foo = {
  bar: {
    baz: {
      nope: 1    
    }
  }
}

if (foo.bar && foo.bar.baz && foo.bar.baz.nope === 1) {
  foo.bar.baz.nope === 2;
}

Can you see the problem on first glance? Probably. But with more noise and stress around me, I couldn't see the bug for a while. "Of course" foo.bar.baz.nope === 2; should be foo.bar.baz.nope === 2;. There is no point to have a line like foo.bar.baz.nope === 2;.

Maybe TypeScript could warn/error for cases like this with an additional setting (similar to settings like noUnusedLocals or noUnusedParameters). Maybe noUnusedExpressions? If expression is used without assigning the value to a variable or without being used inside if () and similar cases, I don't see why anyone would want to write an expression. If someone would write that to invoke some side effects that would be a bad style, too (IMHO).

[DanielRosenwasser]

As a work-around, TSLint's no-unused-expressions rule handles this.

[donaldpipowitch]

Nice. Thank you.

[yortus]

There is some related discussion of side-effect-free expressions in #10814.

[donaldpipowitch]

Cool, thanks for the link.

[mhegazy]

This falls more into the realm of a linter, and would be out of scope for the TS project at the time being.