ts-interface-checker


> 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.