typesafe assertion library for TypeScript 3.7+. 

typesafe assertion library for TypeScript 3.7+

elie.rotenberg

typed-assert

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	// See https://go.microsoft.com/fwlink/?LinkId=827846 to learn about workspace recommendations.

	// Extension identifier format: ${publisher}.${name}. Example: vscode.csharp

	// List of extensions which should be recommended for users of this workspace.

		"VisualStudioExptTeam.vscodeintellicode"

	// List of extensions recommended by VS Code that should not be recommended for users of this workspace.

Object.defineProperty(exports, "__esModule", { value: true });

exports.check = exports.isPromise = exports.isInstanceOf = exports.isOneOfType = exports.isOneOf = exports.isOptionOfType = exports.isArrayOfType = exports.isRecordOfType = exports.isArray = exports.isRecordWithKeys = exports.isRecord = exports.isDate = exports.isString = exports.isNumber = exports.isBoolean = exports.isExactly = exports.isNotUndefined = exports.isNotNull = exports.safeJsonParse = exports.setBaseAssert = exports.assert = exports.defaultAssert = void 0;

const expectedToBe = (type) => `expected to be ${type}`;

exports.defaultAssert = (condition, message) => {

exports.assert = (condition, message) => baseAssert(condition, message);

exports.safeJsonParse = (json) => JSON.parse(json);

function isNotNull(input, message = expectedToBe("not null")) {

    exports.assert(input !== null, message);

function isNotUndefined(input, message = expectedToBe("not undefined")) {

    exports.assert(input !== undefined, message);

function isExactly(input, value, message = expectedToBe(`exactly ${value}`)) {

    exports.assert(input === value, message);

function isBoolean(input, message = expectedToBe("a boolean")) {

    exports.assert(typeof input === "boolean", message);

function isNumber(input, message = expectedToBe("a number")) {

    exports.assert(typeof input === "number", message);

function isString(input, message = expectedToBe("a string")) {

    exports.assert(typeof input === "string", message);

function isDate(input, message = expectedToBe("a Date")) {

    exports.assert(input instanceof Date, message);

function isRecord(input, message = expectedToBe("a record")) {

    exports.assert(typeof input === "object", message);

    for (const key of Object.keys(input)) {

function isRecordWithKeys(input, keys, message = expectedToBe(`a record with keys ${keys.join(", ")}`)) {

exports.isRecordWithKeys = isRecordWithKeys;

function isArray(input, message = expectedToBe("an array")) {

    exports.assert(Array.isArray(input), message);

function isRecordOfType(input, assertT, message = expectedToBe("a record of given type"), itemMessage = expectedToBe("of given type")) {

    for (const item of Object.values(input)) {

function isArrayOfType(input, assertT, message = expectedToBe("an array of given type"), itemMessage = expectedToBe("of given type")) {

function isOptionOfType(input, assertT, message = expectedToBe("option of given type")) {

function isOneOf(input, values, message = expectedToBe(`one of ${values.join(", ")}`)) {

    exports.assert(values.includes(input), message);

function isOneOfType(input, assertT, message = expectedToBe(`one of type`), itemMessage) {

// eslint-disable-next-line @typescript-eslint/no-explicit-any

constructor, message = expectedToBe("an instance of given constructor")) {

    exports.assert(input instanceof constructor, message);

function isPromise(input, message = expectedToBe("a promise")) {

    isInstanceOf(input, Promise, message);

  "description": "typesafe assertion library for TypeScript 3.7+",

    "clean:doc": "rm -rf doc && rm -rf API.md",

    "build:doc": "npm run clean:doc && typedoc && sed s/README.md/API.md/ doc/README.md > API.md && rm -rf doc",

    "build:ts": "npm run clean:ts && tsc -p .",

    "build": "npm run build:doc && npm run build:ts"

    "url": "git+https://github.com/elierotenberg/typed-assert.git"

  "author": "Elie Rotenberg <elie@rotenberg.io>",

    "url": "https://github.com/elierotenberg/typed-assert/issues"

  "homepage": "https://github.com/elierotenberg/typed-assert#readme",

    "@typescript-eslint/eslint-plugin": "^3.2.0",

    "@typescript-eslint/parser": "^3.2.0",

    "@typescript-eslint/eslint-plugin": "^3.6.0",

    "@typescript-eslint/parser": "^3.6.0",

`typed-assert` is a typesafe assertion library implementing the [TS 3.7 Assertion Functions](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions) API, without external dependencies.

`typed-assert` promotes using `unknown` instead of `any` for "untrusted" values, e.g. user input, while still benefiting from incremental typing.

For example, `JSON.stringify` returns `any`, which is not typesafe. With `typed-assert`, we can instead treat the result as `unknown` and gradually check the contents at runtime and still get correct type inference:

const parseConfigFile = (file: string): { readonly a: string, readonly b: number } => {

  const contents = JSON.parse(fs.readFileSync(file, { encoding: 'utf8'})) as unknown;

  // contents is "unknown" instead of any, because we don't trust the input yet

  // contents is "Record<string, unknown>"

### How is it different from chai, jest.expect, etc?

`typed-assert` is both a compile-time and runtime assert library. It leverages the `assertion function` feature of TypeScript to help the typechecker narrow the inferred types. In many cases, this significantly reduces the need to use `any`, and promotes using `unknown` instead.

`typed-assert` comes with a set of common assertions, as well as assertion combinators and utilities.

See [the documentation](./API.md) for a full reference.

function assertCustomType(input: unknown): asserts input is ICustomType {

  .map(x => x % 2 === 0 ? x : null) // x: number

To encourage using asserts when dealing with untrusted JSON input, the following function is also exported:

export const safeJsonParse = (json: string): unknown =>

This library is designed to work in the browser as well as in Node without external dependencies, and by default does not use the `assert` module from the Node stdlib, so it ships with a very basic `assert` implementation:

export type WeakAssert = (input: unknown, message?: string) => void;

export const defaultAssert: WeakAssert = (condition, message) => {

It is however possible to configure the library to use a provided base `assert` function, such as the native `assert` module:

Due to limitations in the typechecker, there are syntactic restrictions in how to define and use type assertion functions. For example, you can not dynamically define an assertion function, even if it looks like a static definition.

function createIsExactly<T>(value: T): (input: unknown) => asserts input is T {

  return function isExactly(input: unknown): asserts input is T {

// Assertions require the call target to be an

//  identifier or qualified name.ts(2776)

For similar reasons, it is not possible to use type-inferred arrow functions to define assertion functions:

const isExactlyNull = (input: unknown): asserts input is null => assert(input === value);

It is however possible to use arrow function with explicit typing of the left-hand operand:

const isExactlyNull: (input: unknown) => asserts input is null = (input) =>

To simplify this pattern, this library also exports the `Assert<Input, Output>` type as defined below:

const isExactlyNull: Assert<null> = (input) => assert(input === null);

For convenience, this library also exports the following types, used internally:

export type SubType<Input, Output> = Output extends Input ? Output : never;

export type Assert<Input = unknown, Output = Input> = (

) => asserts input is SubType<Input, Output>;

export type Check<Input = unknown, Output = Input> = (

const expectedToBe = (type: string): string => `expected to be ${type}`;

export const assert: Assert<boolean, true> = (condition, message) =>

		@@ -97,3 +97,3 @@ "use strict";
		}
		catch { }
		catch (_) { }
		}
		@@ -100,0 +100,0 @@ throw new TypeError(message);

		{
		"name": "typed-assert",
		"version": "1.0.2",
		"version": "1.0.3",
		"description": "typesafe assertion library for TypeScript 3.7+",
		@@ -29,17 +29,17 @@ "main": "build/index.js",
		"devDependencies": {
		"@types/node": "^14.0.13",
		"@types/jest": "^26.0.0",
		"@typescript-eslint/eslint-plugin": "^3.2.0",
		"@typescript-eslint/parser": "^3.2.0",
		"@types/node": "^14.0.22",
		"@types/jest": "^26.0.4",
		"@typescript-eslint/eslint-plugin": "^3.6.0",
		"@typescript-eslint/parser": "^3.6.0",
		"eslint-config-prettier": "^6.11.0",
		"eslint-plugin-import": "^2.21.2",
		"eslint-plugin-prettier": "^3.1.3",
		"eslint": "^7.2.0",
		"jest": "^26.0.1",
		"eslint-plugin-import": "^2.22.0",
		"eslint-plugin-prettier": "^3.1.4",
		"eslint": "^7.4.0",
		"jest": "^26.1.0",
		"prettier": "^2.0.5",
		"ts-jest": "^26.1.0",
		"typedoc": "^0.17.7",
		"typescript": "^3.9.5",
		"ts-jest": "^26.1.2",
		"typedoc": "^0.17.8",
		"typescript": "^3.9.6",
		"typedoc-plugin-markdown": "^2.3.1"
		}
		}
		}

		@@ -127,3 +127,3 @@ # `typed-assert`
		.filter(t.check(t.isNumber))
		.map(x => x % 0 ? x : null)
		.map(x => x % 2 === 0 ? x : null) // x: number
		.filter(t.check(t.isNotNull));
		@@ -130,0 +130,0 @@ // t: number[] = [4, 2]

		@@ -170,3 +170,3 @@ const expectedToBe = (type: string): string => `expected to be ${type}`;
		return;
		} catch {}
		} catch (_) {}
		}
		@@ -173,0 +173,0 @@ throw new TypeError(message);

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