Order of x !== null and typeof x === "object" produce different guards

[kraenhansen]

TypeScript Version: 4.1.0-dev.20200920

Search Terms: type guard unknown null typeof object

Code

declare const something: unknown;

let obj: object;

if (typeof something === "object" && something !== null) {
    // Works fine ✓
    obj = something;
}

if (something !== null && typeof something === "object") {
    // 💥 Type 'object | null' is not assignable to type 'object'.
    obj = something;
}

Expected behavior:

The order in which the type not null guard and a typeof guard is applied, should not matter since none of them have side-effects and the and operator is commutative.

Actual behavior:

When applying a "strict not null" (!== null) type guard before a typeof object guard (typeof .. === "object") the not null guard is forgotten.

Playground Link: Link to playground.

Related Issues: This is the closest I got #32798, but it seems to be caused specifically by a non-strict equal operator.

[IllusionMH]

Duplicate of #28131. Found with null typeof object

[kraenhansen]

Thanks for digging that issue up for me! I will close this and watch the other.