Explain TypeScript code

What this TypeScript sample shows

generics

Result<T> and getJson<T> work for any shape of data. getJson<User> fills in T as User, so result.value is typed as a User.

type narrowing

Result is a union of two object types told apart by ok. After if (!result.ok) return ..., TypeScript knows only the { ok: true; value: T } case is left, so reading result.value compiles.

async/await

Both functions are async, so they return a Promise. await waits for the response, and then for its body, before the next line runs.

modules

export makes the two functions importable from other files. Result and User are not exported, so they stay private to this file.

Good to know

• as T is a promise to the compiler, not a check. If the server sends a different shape, nothing fails until a later line uses a missing field. Validate data that comes from outside your code.
• email?: string means the field may be missing, so its type is string | undefined. The ternary in userLabel handles both cases.
• With strict on, a catch variable has the type unknown, so the code checks err instanceof Error before reading err.message.

Watch out: getJson never throws: every failure becomes { ok: false }. A caller that reads .value without checking ok first gets a compile error, which is the point of the pattern.