TypeScript escape hatches: unsafe operations

turning off type-checking partially or completely

For the following operations, we disable type-checking partially or entirely, either because we know something TypeScript doesn't know, or because we want to disable TypeScript temporarily

assert a value is non-nullish

We remove null and undefined from the value's type:

s // string | null | undefined
s! // string

assert a value conforms to a narrower type

We narrow down the type to a subset:

s // string | null | undefined
s as string // string

Note: in some scenarios, we type assert to a related type, not to a subset. The operation still requires the operation to be likely possible. If the operation is proven to be impossible because the types are incompatible, we must instead type assert to unknown first.

(do not use) assert a value conforms to an incompatible type

s // string | null | undefined
s as unknown as number // number

disable type-checking for a variable

let s: any
s = 3 // type-checker disabled
s = "foo" // type-checker disabled