Should warn against invalid typeof expression

TypeScript Version: 3.2.2

Search Terms: typeof undefined

Code

// This condition will always return 'false' since the types '"string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"' and '42' have no overlap.
typeof 42 === 42

typeof 42 === undefined

typeof 42 === null

Expected behavior:

typeof 42 === undefined

typeof 42 === null

Should warn as what typeof 42 === 42 warns.

Actual behavior:

No warnings

Playground Link: http://www.typescriptlang.org/play/index.html#src=%2F%2F This condition will always return ‘false’ since the types ‘“string” | “number” | “bigint” | “boolean” | “symbol” | “undefined” | “object” | “function”’ and ‘42’ have no overlap. typeof 42 %3D%3D%3D 42 typeof 42 %3D%3D%3D undefined typeof 42 %3D%3D%3D null

Related Issues:

[ajafff]

@fatcerberus TypeScript always allows comparing with null or undefined (because reasons). There’s nothing special about comparing the result of a typeof expression. That’s what @jack-williams means by:

would require special casing the check (with typeof and null), rather than changing the comparability relation.

IMO comparing something with null or undefined shouldn’t be treated different from any other value. If you need to, your type declarations are probably wrong. But that’s already discussed in another issue and was declined for compatibility reasons. Therefore I’ve written a lint rule, see below.

At that point I wonder if this would be better addressed with a lint rule instead.

This check already exists as lint rule.

• TSLint has strict-type-predicates
  • a bit outdated (doesn’t support unknown, bigint or type parameters)
• Fimbullinter has no-useless-predicate
  • supports all types
  • is actually the only rule that doesn’t have its own docs page, yet (I’ll hopefully fix that today)

[Jack-Works]

ah please fix this, I shot on my foot again today