Add promisify (#47)

docs/README.md

@@ -49,4 +49,22 @@ Returns a `Logger` that has `.log`, `.error` and `.warn` methods. Loggins is don
 const l = require('@polkadot/util/logger')('test');
 
 l.log('blah'); // <date>     TEST: blah
+```
+
+## promisify
+
+Wraps an async callback into a `Promise` 
+
+```js
+function promisify (fn: (any) => void, ...params: Array<any>): Promise<any>
+```
+
+
+Wraps the supplied async function `fn` that has a standard JS callback `(error: Error, result: any)` into a `Promise`, passing the supplied parameters. When `error` is set, the Promise is rejected, else the Promise resolves with the `result` value.
+
+```js
+const promisify = require('@polkadot/util/promisify');
+
+await promisify((a, cb) => cb(null, a), true); // resolves with `true`
+await promisify((cb) => cb(new Error('error!'))); // rejects with `error!`
 ```

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@polkadot/util",
-  "version": "0.7.1",
+  "version": "0.8.0",
   "description": "A collection of useful utilities for @polkadot",
   "main": "index.js",
   "keywords": [

src/index.js

@@ -12,11 +12,12 @@ const jsonrpc = require('./jsonrpc');
 const keccak = require('./keccak');
 const logger = require('./logger');
 const number = require('./number');
+const promisify = require('./promisify');
 const u8a = require('./u8a');
 
 /**
   @summary Utility methods for this package are split into groups
 */
 module.exports = Object.assign(
-  {}, array, { assert }, bn, buffer, ext, hex, is, jsonrpc, keccak, { logger }, number, u8a
+  {}, array, { assert }, bn, buffer, ext, hex, is, jsonrpc, keccak, { logger }, number, { promisify }, u8a
 );

src/promisify.js

@@ -0,0 +1,33 @@
+// ISC, Copyright 2017 Jaco Greeff
+// @flow
+
+const assert = require('./assert');
+const isFunction = require('./is/function');
+
+/**
+  @name promisify
+  @signature function promisify (fn: (any) => void, ...params: Array<any>): Promise<any>
+  @summary Wraps an async callback into a `Promise`
+  @description
+    Wraps the supplied async function `fn` that has a standard JS callback `(error: Error, result: any)` into a `Promise`, passing the supplied parameters. When `error` is set, the Promise is rejected, else the Promise resolves with the `result` value.
+  @example
+    const promisify = require('@polkadot/util/promisify');
+
+    await promisify((a, cb) => cb(null, a), true); // resolves with `true`
+    await promisify((cb) => cb(new Error('error!'))); // rejects with `error!`
+*/
+module.exports = function promisify (fn: (any) => void, ...params: Array<any>): Promise<any> {
+  assert(isFunction(fn), 'Expected function input to promisify');
+
+  return new Promise((resolve, reject) => {
+    fn.apply(null, [].concat(params, [
+      (error: Error, result: any): void => {
+        if (error) {
+          reject(error);
+        } else {
+          resolve(result);
+        }
+      }
+    ]));
+  });
+};

src/promisify.spec.js

@@ -0,0 +1,43 @@
+// ISC, Copyright 2017 Jaco Greeff
+
+const { promisify } = require('./index');
+
+describe('promisify', () => {
+  it('requires a function to promisify', () => {
+    expect(
+      () => promisify()
+    ).toThrow(/Expected function input/);
+  });
+
+  it('handles functions with no parameters (resolve)', () => {
+    const fn = (cb) => cb(null, [true, 'test', 1]);
+
+    return promisify(fn).then((result) => {
+      expect(result).toEqual([true, 'test', 1]);
+    });
+  });
+
+  it('handles functions with no parameters (reject)', () => {
+    const fn = (cb) => cb(new Error('test reject'));
+
+    return promisify(fn).catch((error) => {
+      expect(error.message).toEqual('test reject');
+    });
+  });
+
+  it('handles functions with parameters (resolve)', () => {
+    const fn = (a, b, c, cb) => cb(null, [a, b, c]);
+
+    return promisify(fn, 2, false, null).then((result) => {
+      expect(result).toEqual([2, false, null]);
+    });
+  });
+
+  it('handles functions with parameters (reject)', () => {
+    const fn = (a, b, c, cb) => cb(new Error(`test reject: ${a},${b},${c}`));
+
+    return promisify(fn, 3, 'string', true).catch((error) => {
+      expect(error.message).toEqual('test reject: 3,string,true');
+    });
+  });
+});