SInon yields demo

index.js

'use strict';

const fs = require('fs');

exports.readJsonFile = function (file, callback) {
  fs.readFile(file, 'utf8', (err, content) => {
    if (err) {
      callback(err);
      return;
    }

    let json;
    try {
      json = JSON.parse(content);
    } catch (e) {
      callback(e);
      return;
    }

    callback(null, json);
  });
};

package.json

{
  "name": "sinon-yields-demo",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "mocha test.js",
    "watch": "mocha test.js --watch"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "mocha": "^5.0.1",
    "sinon": "^4.4.2"
  }
}

test.js

/*eslint-env mocha*/
'use strict';

const assert = require('assert');
const sinon = require('sinon');
const fs = require('fs');
const api = require('./');

describe('readJsonFile', () => {
  const sandbox = sinon.createSandbox();
  
  beforeEach(() => {
    sandbox.stub(fs, 'readFile');
  });

  afterEach(() => {
    sandbox.restore();
  });

  it('calls fs.readFile with utf8 encoding', () => {
    api.readJsonFile('some-file.json', () => {});

    sinon.assert.calledWith(fs.readFile, 'some-file.json', 'utf8');
  });

  it('yields parsed file content', () => {
    fs.readFile.yields(null, '{"some":"json"}');
    const callback = sinon.spy();

    api.readJsonFile('x.json', callback);

    sinon.assert.calledWith(callback, null, { some: 'json' });
  });

  it('yields error from readFile', () => {
    const error = new Error('Oh noes!');
    fs.readFile.yields(error);
    const callback = sinon.spy();

    api.readJsonFile('x.json', callback);

    sinon.assert.calledWith(callback, error);
  });

  it('yields error from JSON.parse', () => {
    fs.readFile.yields(null, '{"some":invalid json}');
    const callback = sinon.spy();

    api.readJsonFile('x.json', callback);

    sinon.assert.calledWithMatch(callback, { name: 'SyntaxError' });
  });

  it('does not yield error thrown by callback', () => {
    fs.readFile.yields(null, '{"some":"json"}');
    const callback = sinon.spy((err, content) => {
      if (content) {
        throw new Error('Oups!');
      }
    })

    assert.throws(() => {
      api.readJsonFile('x.json', callback);
    }, /Error: Oups!/);

    sinon.assert.calledOnce(callback);
    sinon.assert.calledWith(callback, null, { some: 'json' });
  });

});