add bind option

index.js

@@ -1,8 +1,8 @@
 'use strict';
 
-var processFn = function (fn, P, opts) {
+var processFn = function (fn, P, opts, bind) {
 	return function () {
-		var that = this;
+		var that = bind || this;
 		var args = new Array(arguments.length);
 
 		for (var i = 0; i < arguments.length; i++) {
@@ -39,6 +39,11 @@ var pify = module.exports = function (obj, P, opts) {
 
 	opts = opts || {};
 	var exclude = opts.exclude || [/.+Sync$/];
+	var bind = opts.bind === false ? false : (opts.bind || obj);
+
+	if (bind === true) {
+		bind = obj;
+	}
 
 	var filter = function (key) {
 		var match = function (pattern) {
@@ -59,7 +64,26 @@ var pify = module.exports = function (obj, P, opts) {
 	return Object.keys(obj).reduce(function (ret, key) {
 		var x = obj[key];
 
-		ret[key] = typeof x === 'function' && filter(key) ? processFn(x, P, opts) : x;
+		if (typeof x === 'function') {
+			if (filter(key)) {
+				x = processFn(x, P, opts, bind);
+			}
+
+			ret[key] = x;
+		} else if (bind) {
+			Object.defineProperty(ret, key, {
+				enumerable: true,
+				configurable: true,
+				get: function () {
+					return bind[key];
+				},
+				set: function (val) {
+					bind[key] = val;
+				}
+			});
+		} else {
+			ret[key] = x;
+		}
 
 		return ret;
 	}, ret);

optimization-test.js

@@ -40,5 +40,5 @@ sut.unicorn().then(function () {
 	});
 }).catch(function (err) {
 	console.log(err.stack);
-	process.exit(1);
+	process.exit(1); // eslint-disable-line xo/no-process-exit
 });

test.js

@@ -48,6 +48,69 @@ test('wrap core method', async t => {
 	t.is(JSON.parse(await fn(fs.readFile)('package.json')).name, 'pify');
 });
 
+test('binds to the original object by default', async t => {
+	const obj = {
+		x: 'foo',
+		y: function (cb) {
+			setImmediate(() => cb(null, this.x));
+		}
+	};
+
+	const pified = fn(obj);
+	obj.x = 'bar';
+
+	t.is(await pified.y(), 'bar');
+	t.is(pified.x, 'bar');
+});
+
+test('bind:true will bind to the original object', async t => {
+	const obj = {
+		x: 'foo',
+		y: function (cb) {
+			setImmediate(() => cb(null, this.x));
+		}
+	};
+
+	const pified = fn(obj, {bind: true});
+	obj.x = 'bar';
+
+	t.is(await pified.y(), 'bar');
+	t.is(pified.x, 'bar');
+});
+
+test('bind:otherObj will bind to otherObj', async t => {
+	const obj = {
+		x: 'foo',
+		y: function (cb) {
+			setImmediate(() => cb(null, this.x));
+		}
+	};
+
+	const otherObj = {
+		x: 'baz'
+	};
+
+	const pified = fn(obj, {bind: otherObj});
+
+	t.is(await pified.y(), 'baz');
+});
+
+test('bind:false creates a copy', async t => {
+	const obj = {
+		x: 'foo',
+		y: function (cb) {
+			setImmediate(() => cb(null, this.x));
+		}
+	};
+
+	const pified = fn(obj, {bind: false});
+
+	obj.x = 'bar';
+
+	t.is(await pified.y(), 'foo');
+	t.is(pified.x, 'foo');
+});
+
 test('module support', async t => {
 	t.is(JSON.parse(await fn(fs).readFile('package.json')).name, 'pify');
 });