A collection of generic type guards to check runtime variables in TypeScript.
- Install the npm package ts-guardsnpm i ts-guards
- Use the package;let x = "a string";// Type of x inferred inside the if statement as: let x: stringifprimitiveType console;// Type of x inferred after the call as:// Throws an error if type doesn't matchasserts;// Properties of object inferred (if object does not have an x and a y property, it throws an error)asserts;// Type of x inferred inside the if statement as: let x: stringif
TypeScript helps only with compile time validation, you need to check anything coming from IO at runtime. TypeScript runtime validation relies upon type guards.
Type guards take a parameter
x as unknown, denoting variables whose type we do not know.
There are two styles of validation: one relying on
x is T; another relying on
asserts x is T.
The former can be used in conditional cases (returns a boolean), the latter for input validation (throws an error).
One might consider that functions given a wrong parameter can’t answer the question they’re supposed to, hence they should throw an error, hence the
asserts x is T style (error throwing).
asserts x is T style functions rely upon and have a
x is T counterpart. Both validation styles trigger TypeScript type inference.
In some cases, type guards may take a parameter
x: T to catter for output type inference.