24 — Narrowing

One line: After a test that only some members of a union can pass, the checker shrinks the variable's type inside that branch; the chapter is about which tests it understands, which it takes on trust, and how to make it prove no case was forgotten.

Narrowing makes a union usable: the checker reads typeof, in, instanceof and equality tests and shrinks the type in each branch. The chapter starts with those built-in tests and with truthiness, the test that also turns away 0 and "". It then builds unions meant to be narrowed, discriminated by a tag, and uses never to catch a forgotten case. It ends with the tests you write yourself, type predicates and assertion functions, which the checker trusts without looking, and with satisfies, as and annotations.

The pages

Page	Level	In one line	Status
Narrowing	201	After if (typeof value === "string") return, the rest of the function knows value is not a string; but typeof value === "object" also lets null through, so reading value.tags in that branch is TS18047, 'value' is possibly 'null'.	stub
Truthiness narrowing	201	if (count) narrows number | null to number, which reads as 'not null', but at run time 0 fails the test too: label(0) prints no count, and only the type after the if, still number | null, hints that a number can arrive there.	stub
Discriminated unions	201	A union narrows on a tag only when each member's tag is a literal type: with kind: "circle" and kind: "square", switch (shape.kind) gives each case its member, even through const { kind } = shape; with kind: string, reading radius is TS2339.	stub
Exhaustiveness checking	201	Once every member of a union has its case, what reaches default is never; assign it to a never variable there, and adding a triangle member later turns the forgotten case into TS2322 on that line, naming the triangle type.	stub
Type predicates	201	The checker takes a type predicate on trust: isString(value): value is string that returns true for 42 compiles cleanly, so input.toUpperCase() passes the check and throws a TypeError at run time.	stub
satisfies, as and annotations	201	An annotation checks an object and forgets its details, satisfies checks it and keeps them, and as barely checks: { red: [255, 0, 0] } as Palette compiles without the required green, which satisfies Palette reports as TS2741.	stub

Boundaries

Unions and literal types are introduced in Everyday types; the assertions that lie outright, as and the postfix !, are shown failing in Where types lie.