23 — Everyday types

One line: Each everyday type names a set of values, and the surprises follow from that: a union of two object types lets you use fewer members, a readonly property guards only one path to the object, and a tuple can still push.

This chapter covers the types written in ordinary code, starting with the smallest: primitive and literal types, then arrays and tuples, object types, and the two ways to combine types, | and &. Once shapes can be written, it compares the two ways to name them, type and interface, and turns to function types and overloads. The last three pages cover null under strictNullChecks; any, unknown and never, the types at the edges; and enums, which this library replaces with erasable unions.

The pages

Page	Level	In one line	Status
Primitive and literal types	101	A literal type has exactly one value: const alone = "dark" has the type "dark" and passes where "dark" | "light" is expected, but the same string inside { mode: "dark" } widens to string and is rejected until you add as const.	stub
Arrays and tuples	101	A tuple type is an array with a type per position, not a fixed-length array: pair.push(99) on a [string, number] compiles and makes length 3, yet tsc still rejects pair[2]; only readonly [string, number] refuses the push.	stub
Object types	101	readonly on a property stops writes through that type only: assign the object to a variable typed { id: number } and loose.id = 2 compiles, changes the original, and tsc still rejects fixed.id = 3 as TS2540.	stub
Union and intersection types	101	| and & combine sets of values, not lists of members: on Cat | Dog only the shared name is usable and either.meow() is TS2339, Cat & Dog has every member of both, and string & number is never.	stub
Type aliases and interfaces	201	Declare interface Box twice and the two declarations merge into one type; declare type Crate twice and it is TS2300, a duplicate. And extends reports a conflicting property (TS2430), while & accepts it and quietly makes it never.	stub
Function types	201	Callers see only the overload signatures, never the implementation's: pad("a", 3) is rejected although (value: string | number, width?: number) would accept it, and a callback may take fewer parameters than its type declares but not more.	stub
strictNullChecks	101	Under strictNullChecks, on by default in TypeScript 7, names.find(...) is string | undefined and returning it as a string is TS2322; switch the check off and the same file compiles, then fails at run time reading length of undefined.	stub
any, unknown and never	201	any and unknown both accept every value, but loose.toFixed(2) on an any compiles while the same call on an unknown is TS18046; never has no values, and even any cannot be assigned to it.	stub
Enums	201	A numeric enum compiles to an object keyed both ways, so Object.keys of a two-member enum gives four keys, 0, 1, Up and Down; Node's type stripping refuses an enum, and an as const object plus a union type does the same job.	stub

Boundaries

Taking a union apart again, with typeof, tags and predicates, is the Narrowing chapter; which of these types may be assigned to which is Type compatibility.