Jest-nest NPM | npm.io

jest-nest

Jest mocks for curried, chained functions and nested objects

Create mock for curried function with any number of parameters
Create mock for chained function of any depth
Create mock for objects that return (deeply) nested objects and specify return values at any path
Easy to use expectation

With this you can write:

const mock = nest.obj().mockReturnValueAt('foo', 'bar', nest.curry(3));

expect(mock).toHaveBeenNestedCalledWith(nest.args.foo('a', 'b').bar('c')('d', 'e')('f'));

Instead of:

const deeperNestedFunction = jest.fn();
const nestedFunction = jest.fn().mockReturnValue(deeperNestedFunction);
const nestedObject = { bar: jest.fn().mockReturnValue(nestedFunction) };
const mock = { foo: jest.fn().mockReturnValue(nestedObject) };

expect(mock).toHaveBeenCalledWith('a', 'b');
expect(nestedObject).toHaveBeenCalledWith('c');
expect(nestedFunction).toHaveBeenCalledWith('d', 'e');
expect(deeperNestedFunction).toHaveBeenCalledWith('f');

Installation

$ npm install jest-nest

Configuration

Create a setup file:

import { init } from 'jest-nest';

init();

And load it in your jest config:

import { Config } from 'jest';

const config: Config = {
  ...
  setupFilesAfterEnv: ['./test/setup.ts'],
};

export default config;

Usage

Curried functions

it('expects curried function call', () => {
  const mockImplementation = (a, b, c, d) => 'my return value';
  const curryFn = nest.curry(mockImplementation);

  const result = nestedFn('a', 'b')({ foo: 'bar' })('c');

  // Make any expectation against curryFn.uncurried like you would with any jest.Mock
  expect(curryFn.uncurried).toHaveBeenCalledWith('a', 'b', { foo: expect.any(String) }, 'c');
  // Or make expectations on how the mock was called
  expect(curryFn).toHaveBeenNestedCalledWith(nest.args('a', 'b')({ foo: expect.any(String) })('c'));

  expect(result).toBe('my return value');
});

or without an implementation

it('expects curried function call', () => {
  const arity = 4;
  const curryFn = nest.curry(arity);

  const result = nestedFn('a', 'b')({ foo: 'bar' })('c');

  expect(curryFn.uncurried).toHaveBeenCalledWith('a', 'b', { foo: expect.any(String) }, 'c');
});

or with a curried function as implementation

it('expects curried function call', () => {
  import myCurriedFunc from './myFunc';

  // Because the function is curried, the arity cannot be derived automatically
  const curryFn = nest.curry(myCurriedFunc, 4);

  const result = nestedFn('a', 'b')({ foo: 'bar' })('c');

  // Expectations can be made on full chains
  expect(curryFn.uncurried).toHaveBeenCalledWith('a', 'b', { foo: expect.any(String) }, 'c');
  // or make expectations on how the mock was (partially) called
  expect(curryFn).toHaveBeenNestedCalledWith(nest.args('a', 'b'));
});

Chained functions

For nested functions with a fixed depth of nesting and possibly with optional arguments, the nesting mock can be used.

it('expects nested function call', () => {
  const nestedFn = nest.fn(
    3, // depth, optional, default: 2
    () => 'my return value', // tail implementation, optional
  );

  const result = nestedFn('a', 'b')({ foo: 'bar' })('c');

  expect(nestedFn).toHaveBeenNestedCalledWith(nest.args('a', 'b')({ foo: expect.any(String) })('c'));
  expect(result).toBe('my return value');
});

(Nested) objects

The basic Object mock is an indexer of functions that return an object mock (i.e. you can call any string/symbol on the mock and it will return a new mock).

it('expects a nested object', () => {
  const mock = nest.obj();

  mock.foo('a').bar('b', 'c').baz('d');

  expect(mock).toHaveBeenNestedCalledWith(nest.args.foo('a').bar('b', 'c').baz('d'));
});

The Object mock can be configured to return given values at given paths:

it('sets values at paths' () => {
  const mock = nest
    .obj()
    .mockReturnValueAt('foo', 'bar', 42) // mock.foo(...).bar(...) returns 42
    .mockResolvedValueAt('foo', 'baz', 42) // mock.foo(...).bar(...) returns a resolved promise with value 42
    .mockRejectedValueAt('bar', 'foo', 'oops') // mock.bar(...).foo(...) returns a rejected promise with value 'oops'
    .mockGetValueAt('bar', 'baz', 42); // mock.bar(...).baz gets a property with value 42
});

or set a given implementation at a given path:

it('sets an implementation at a path', () => {
  const mock = nest
    .obj()
    .mockImplementationAt('isEven', (value: number) => value % 2 === 0) // mock.isEven(2) returns true since 2 is even
    .mockImplementationAt('adderOf', 'add', function (this: CallState, value: number) {
      const [adderOfArgs] = this.callPath; // Gets the arguments of the adderOf call
      const adderValue = adderOfArgs[1]; // Gets the first argument (argument at index 0 is the name of the property ('adderOf'))

      return adderValue + value;
    }); // mock.adderOf(5).add(2) returns 7
});

By default the object mock returns an object mock when calling a property for which no path has been set. This behaviour can be overridden:

it('sets the strictness of the object mock', () => {
  const mock = nest
    .obj() // mock.foo() returns an object mock
    .strict() // mock.foo returns 'undefined', since no return value has been set for path 'foo'
    .explicit() // alias for mock.strict()
    .implicit(); // mock.foo() returns an object mock
});

Combined mocks

Nest mocks, when combined, allow expectations over the entire chain:

it('supports expectations across multiple nest mocks', () => {
  const mock = nest.obj().mockReturnValueAt(
    'foo',
    'bar',
    nest.curry(() => nest.obj(), 4),
  );

  mock.foo('a').bar('b')('c')('d', 'e')('f').baz('g');

  expect(mock).toHaveBeenCalledTimes(1);
  expect(mock).toHaveBeenNestedCalledWith(nest.args.foo('a').bar('b')('c')('d', 'e')('f').baz('g'));
});

Api reference

Mocks

interface CallState {
      readonly callPath: ReadonlyArray<ReadonlyArray<any>>;
}

export type NestingMock = jest.Mock & CallState;

export type CurryMock = NestingMock & {
  uncurried: jest.Mock;
};

export type ObjectMock<Shape, Strict extends boolean> = NestingMock & {
  mockImplementationAt: (...path: string[], implementation: (this: CallState, ...args: any[]) => any): ObjectMock<NewShape, Strict>;
  mockReturnValueAt: (...path: string[], value: any) => any): ObjectMock<NewShape, Strict>;
  mockResolvedValueAt: (...path: string[], value: any): ObjectMock<NewShape, Strict>;
  mockRejectedValueAt: (...path: string[], value: any): ObjectMock<NewShape, Strict>;
  mockGetValueAt: (...path: string[], value: any): ObjectMock<NewShape, Strict>;
  mockStrict: (): ObjectMock<Shape, true>;
  mockImplicit: (): ObjectMock<Shape, false>;
} & Shape & Record<string | number | symbol, Strict extends true ? never : jest.Mock<ObjectMock<{}, Strict>>>

export declare function fnCurried(mockImplementation: (...args: any[]) => any, arity?: number): CurryMock;
export declare function fnCurried(arity: number): CurryMock;

export declare function fnNested(depth?: number, tailImplementation?: (...args: any[]) => any): NestingMock;

export declare function objNested(): ObjectMock<{}, false>;

Expectations

namespace jest {
  interface Matchers<R> {
    toHaveBeenNestedCalledWith(nestedArgs: any[][] | NestingArgs): R;
  }
}

export type NestingArgs = {
  (...args: any[]): NestingArgs;
  args: any[][];
} & Record<string | number | symbol, (...args: any[]) => NestingArgs>;

export declare const nestingArgs: NestingArgs;

Global exports

declare global {
  var nest: {
    fn: typeof fnNested;
    chain: fnNested;
    curry: typeof fnCurried;
    obj: typeof objNested;
    args: typeof nestingArgs;
  };
}