promiser.js

const createPromise = require('./promiser.js');

// How to create promises from functions. const fs = require('fs');

createPromise(fs.readFile)('test.js', 'utf8'). then(() => console.log('SUCCESSFUL SUCCESS (function)')). catch(() => { throw new Error('FAILED SUCCESS (function)');});

createPromise(fs.readFile)('non-existent-file', 'utf8'). then(() => { throw new Error('FAILED FAILURE (function)');}). catch(() => console.log('SUCCESSFUL FAILURE (function)'));

// How to create promise from methods. class TestClass { constructor(successStr, failStr, success) { this.failStr = failStr; this.successStr = successStr; this.success = success; } a(errFirstCallBack) { if (this.success) { errFirstCallBack(null, this.successStr); } else { errFirstCallBack(this.failStr, null); } } }

const testClassSuccess = new TestClass('SUCCESSFUL SUCCESS (method)', 'FAILED SUCCESS (method)', true);

// Remember to pass context to createPromise createPromise(testClassSuccess.a, testClassSuccess)(). then(console.log). catch(console.error);

const testClassFailure = new TestClass('FAILED FAILURE (method)', 'SUCCESSFUL FAILURE (method)', false);

createPromise(testClassFailure.a, testClassFailure)(). then(console.log). catch(console.error);