src: Remove the Promisify function (#354)

BREAKING CHANGE: Remove the Promisify function from the CircuitBreaker factory

* Node has its own built-in promisify function that can be used instead.

fixes #352

README.md

@@ -205,14 +205,15 @@ circuit.on('fallback',
 The `opossum` API returns a `Promise` from `CircuitBreaker.fire()`.
 But your circuit action - the async function that might fail -
 doesn't have to return a promise. You can easily turn Node.js style
-callback functions into something `opossum` understands by using
-`circuitBreaker.promisify()`.
+callback functions into something `opossum` understands by using the built in
+Node core utility function `util.promisify()` .
 
 ```javascript
 const fs = require('fs');
+const { promisify } = require('util');
 const circuitBreaker = require('opossum');
 
-const readFile = circuitBreaker.promisify(fs.readFile);
+const readFile = promisify(fs.readFile);
 const breaker = circuitBreaker(readFile, options);
 
 breaker.fire('./package.json', 'utf-8')

index.js

@@ -69,21 +69,6 @@ function factory (action, options) {
   return lastCircuit;
 }
 
-/**
- * Given a function that receives a callback as its last argument,
- * and which executes that function, passing as parameters `err` and `result`,
- * creates an action that returns a promise which resolves when the function's
- * callback is executed.
- * @function factory.promisify
- *
- * @param {Function} action A Node.js-like asynchronous function
- * @example
- *     const fs = require('fs');
- *     const readFilePromised = circuitBreaker.promisify(fs.readFile);
- *     const breaker = circuitBreaker(readFilePromised);
- */
-factory.promisify = require('./lib/promisify');
-
 /**
  * Get the Prometheus metrics for all circuits.
  * @function factory.metrics
@@ -115,7 +100,7 @@ Object.defineProperty(factory, 'stats', {
  * @return {Iterator} an <code>Iterator</code> of all available circuits
  */
 factory.circuits = CircuitBreaker.circuits;
-  
+
 module.exports = exports = factory;
 // Allow use of default import syntax in TypeScript
 module.exports.default = factory;

test/test.js

@@ -2,14 +2,14 @@
 
 const browser = require('./browser/browser-tap');
 const test = require('tape');
+const {promisify} = require('util');
 const circuit = require('../');
 
 browser.enable();
 
 test('api', t => {
   const breaker = circuit(passFail);
   t.ok(breaker, 'CircuitBreaker');
-  t.ok(circuit.promisify, 'CircuitBreaker.promisify');
   t.ok(breaker.fire, 'CircuitBreaker.fire');
   t.notOk(breaker.opened, 'CircuitBreaker.opened');
   t.notOk(breaker.halfOpen, 'CircuitBreaker.halfOpen');
@@ -167,19 +167,19 @@ test('Works with non-functions', t => {
 
 test('Works with callback functions', t => {
   t.plan(1);
-  const promisified = circuit.promisify(callbackFunction);
+  const promisified = promisify(callbackFunction);
   const breaker = circuit(promisified);
 
   breaker.fire(3, 4)
-    .then(arg => t.equals(arg, 7, 'CircuitBreaker.promisify works'))
+    .then(arg => t.equals(arg, 7, 'Works with a Promisified Callback'))
     .then(_ => breaker.shutdown())
     .then(t.end)
     .catch(t.fail);
 });
 
 test('Works with callback functions that fail', t => {
   t.plan(1);
-  const promisified = circuit.promisify(failedCallbackFunction);
+  const promisified = promisify(failedCallbackFunction);
   const breaker = circuit(promisified);
 
   breaker.fire(3, 4)