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+# ts-interface-checker
+
+[![Build Status](https://travis-ci.org/gristlabs/ts-interface-checker.svg?branch=master)](https://travis-ci.org/gristlabs/ts-interface-checker)
+[![npm version](https://badge.fury.io/js/ts-interface-checker.svg)](https://badge.fury.io/js/ts-interface-checker)
+
+
+> Runtime library to validate data against TypeScript interfaces.
+
+This package is the runtime support for validators created by
+[ts-interface-builder](https://github.com/gristlabs/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<Square>};
+
+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.