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    Runtime library to validate data against TypeScript interfaces.

    This package is the runtime support for validators created by ts-interface-builder. It allows validating data, such as parsed JSON objects received over the network, or parsed JSON or YAML files, to check if they satisfy a TypeScript interface, and to produce informative error messages if they do not.


    npm install --save-dev ts-interface-builder
    npm install --save ts-interface-checker


    Suppose you have a TypeScript file defining an interface:

    // foo.ts
    interface Square {
      size: number;
      color?: string;

    The first step is to generate some code for runtime checks:

    `npm bin`/ts-interface-builder foo.ts

    It produces a file like this:

    // foo-ti.js
    import * as t from "ts-interface-checker";
    export const Square = t.iface([], {
      "size": "number",
      "color": t.opt("string"),

    Now at runtime, to check if a value satisfies the Square interface:

    import fooTI from "./foo-ti";
    import {createCheckers} from "ts-interface-checker";
    const {Square} = createCheckers(fooTI);
    Square.check({size: 1});                  // OK
    Square.check({size: 1, color: "green"});  // OK
    Square.check({color: "green"});           // Fails with "value.size is missing"
    Square.check({size: 4, color: 5});        // Fails with "value.color is not a string"

    Note that ts-interface-builder is only needed for the build-time step, and ts-interface-checker is needed at runtime. That's why the recommendation is to npm-install the former using --save-dev flag and the latter using --save.

    Checking method calls

    If you have an interface with methods, you can validate method call arguments and return values:

    // greet.ts
    interface Greeter {
      greet(name: string): string;

    After generating the runtime code, you can now check calls like:

    import greetTI from "./greet-ti";
    import {createCheckers} from "ts-interface-checker";
    const {Greeter} = createCheckers(greetTI);
    Greeter.methodArgs("greet").check(["Bob"]);     // OK
    Greeter.methodArgs("greet").check([17]);        // Fails with " is not a string"
    Greeter.methodArgs("greet").check([]);          // Fails with " is missing"
    Greeter.methodResult("greet").check("hello");   // OK
    Greeter.methodResult("greet").check(null);      // Fails with "value is not a string"

    Type suites

    If one type refers to a type defined in another file, you need to tell the interface checker about all type names when you call createCheckers(). E.g. given

    // color.ts
    export type Color = RGB | string;
    export type RGB = [number, number, number];
    // shape.ts
    import {Color} from "./color";
    export interface Square {
      size: number;
      color?: Color;

    the produced files color-ti.ts and shape-ti.ts do not automatically refer to each other, but expect you to relate them in createCheckers() call:

    import color from "./color-ti";
    import shape from "./shape-ti";
    import {createCheckers} from "ts-interface-checker";
    const {Square} = createCheckers(shape, color);    // Pass in all required type suites.
    Square.check({size: 1, color: [255,255,255]});

    Strict checking

    You may check that data contains no extra properties. Note that it is not generally recommended as it this prevents backward compatibility: if you add new properties to an interface, then older code with strict checks will not accept them.

    Following on the example above:

    Square.strictCheck({size: 1, color: [255,255,255], bg: "blue"});    // Fails with is extraneous
    Square.strictCheck({size: 1, color: [255,255,255,0.5]});            // Fails with ...value.color[3] is extraneous

    Type guards

    Standard Checker objects do the type checking logic, but are unable to make the TypeScript compiler aware that an object of unknown type implements a certain interface.

    Basic code:

    const unk: unknown = {size: 1, color: "green"};
    // Type is unknown, so TypeScript will not let you access the members.
    console.log(unk.size); // Error: "Object is of type 'unknown'"

    With a Checker available:

    import fooTI from "./foo-ti";
    import {createCheckers} from "ts-interface-checker";
    const {Square} = createCheckers(fooTI);
    const unk: unknown = {size: 1, color: "green"};
    if (Square.test(unk)) {
      // unk does implement Square, but TypeScript is not aware of it.
      console.log(unk.size); // Error: "Object is of type 'unknown'"

    To enable type guard functionality on the existing test, and strictTest functions, Checker objects should be cast to CheckerT<> using the appropriate type.

    Using CheckerT<>:

    import {Square} from "./foo";
    import fooTI from "./foo-ti";
    import {createCheckers, CheckerT} from "ts-interface-checker";
    const {Square} = createCheckers(fooTI) as {Square: CheckerT<Square>};
    const unk: unknown = {size: 1, color: "green"};
    if (Square.test(unk)) {
      // TypeScript is now aware that unk implements Square, and allows member access.

    Type assertions

    CheckerT<> will eventually support type assertions using the check and strictCheck functions, however, this feature is not yet fully working in TypeScript.


    npm i ts-interface-checker

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    • paulfitz
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