Jestサンプルのテスト対象

app.spec.ts

// (1): モックを定義 (1)
const mockFileData = `test\n is\n spec.`;
jest.mock('fs', () => {
    return {
        readFileSync: jest.fn(() => mockFileData),
        readFile: jest.fn((path, opt, cb) => {
            cb(null, mockFileData);
        });
    };
});
// (2): 検証用((4),(5))にモック読み込む
import * as fs from 'fs';

// (3): (1)を利用するモジュールを読み込む
import {LineCount, LineCountAsync, LineCountDiff} from './app';

describe('test', () => {

    beforeEach(() => {
        fs.readFileSync.mockClear();
    });

    const path = 'dummy';

    it('LineCount()', () => {
        const result = LineCount(path);

        expect(result).toBe(3);
        expect(fs.readFileSync.mock.calls.length).toBe(1); // (4): モックが呼ばれたか検証
        expect(fs.readFileSync.mock.calls[0][0]).toBe(path); // (5): モックが呼ばれた際の引数を検証
    });

    it('LineCountAsync', () => {
        return LineCountAsync(path).then(result => {
            expect(result).toBe(3);
        });
    });

    it('LineCount() // ファイルが空', () => {
        fs.readFileSync.mockImplementation(() => ''); // 空文字を返す振る舞いを定義
        const result = LineCount(path);
        expect(result).toBe(1); // テストに失敗します。 
    });

    it('LineCount() // ファイルが空でない', () => {
        fs.readFileSync.mockImplementation(() => 'hello'); // 文字列を返す振る舞いを定義
        const result = LineCount(path);
        expect(result).toBe(1);
    });

    it('LineCountDiff()', () => {
        const path2 = 'dummy2';
        const generator = function* (e) { yield *e; };
        const mockFn = generator(['1\n2\n3', '1']);

        fs.readFileSync.mockImplementation(() =>  mockFn.next().value);

        const result = LineCountDiff(path, path2);
        expect(result).toBe(2);
    });

});

app.ts

import {promisify} from 'util';
import * as fs from 'fs';

const readFilePromise = promisify(fs.readFile);
        
export function LineCount (path: string): number {
    const data = fs.readFileSync(path, 'utf8');
    return data.split('\n').length;
}
    
export async function LineCountAsync (path: string): Promise<number> {
    const data = await readFilePromise(path, 'utf8');
    return data.split('\n').length;
}       
        
export function LineCountDiff (path1: string, path2: string): number {
    const data1 = fs.readFileSync(path1, 'utf8');
    const data2 = fs.readFileSync(path2, 'utf8');
    return data1.split('\n').length - data2.split('\n').length;
}           
        
export default { LineCount, LineCountAsync, LineCountDiff };