National Preventative Mechanism

    ts-guards
    TypeScript icon, indicating that this package has built-in type declarations

    0.5.1 • Public • Published

    Type Guards

    A collection of generic type guards to check runtime variables in TypeScript.

    How-to?

    • Install the npm package ts-guards
      npm i ts-guards
    • Use the package
      import { asserts, primitiveType } from 'ts-guards';
       
      let x = "a string";
       
      // Type of x inferred inside the if statement as: let x: string
      if(primitiveType.isString(x)) { console.log(x); }
       
      // Type of x inferred after the call as: 
      // Throws an error if type doesn't match
      asserts.isString(x);
       
      // Properties of object inferred (if object does not have an x and a y property, it throws an error)
      asserts.areObjectPropertiesOf({ x: "y", y: "x" }, ["x", "y"]);
       
      // Type of x inferred inside the if statement as: let x: string
      if(isLiteral(x, "x" as const)) { x } 
       
      // Type of x inferred inside the if statement as: let x: "x" | 1 | "y" | "z"
      if(isLiteralType(x, new Set([ "x", 1, "y", "z" ] as const))) { x } 

    Why?

    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).

    All of 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.

    Install

    npm i ts-guards

    DownloadsWeekly Downloads

    197

    Version

    0.5.1

    License

    MIT

    Unpacked Size

    21.4 kB

    Total Files

    18

    Last publish

    Collaborators

    • matthieubosquet