ts-interface-checker

Build Status npm version

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.

Installation

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

Usage

Suppose you have a TypeScript file defining an interface: typescript // foo.ts interface Square { size: number; color?: string; }

The first step is to generate some code for runtime checks: bash `npm bin`/ts-interface-builder foo.ts

It produces a file like this: ```typescript // 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: ```typescript 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: typescript // greet.ts interface Greeter { greet(name: string): string; }

After generating the runtime code, you can now check calls like: ```typescript 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 "value.name is not a string" Greeter.methodArgs("greet").check([]); // Fails with "value.name 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

typescript // color.ts export type Color = RGB | string; export type RGB = [number, number, number];

typescript // 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: ```typescript 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: typescript Square.strictCheck({size: 1, color: [255,255,255], bg: "blue"}); // Fails with value.bg 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: typescript 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: ```typescript 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<>: ```typescript import {Square} from "./foo"; import fooTI from "./foo-ti"; import {createCheckers, CheckerT} from "ts-interface-checker";

const {Square} = createCheckers(fooTI) as {Square: CheckerT};

const unk: unknown = {size: 1, color: "green"};

if (Square.test(unk)) { // TypeScript is now aware that unk implements Square, and allows member access. console.log(unk.size); } ```

Type assertions

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