Initial implementation.

.eslintrc

@@ -3,6 +3,14 @@
 
 	"extends": "@ljharb",
 
+	"env": {
+		"es6": true
+	},
+
 	"rules": {
+		"id-length": [2, { "max": 30 }],
+		"max-statements": [2, 16],
+		"no-magic-numbers": 0,
+		"operator-linebreak": [2, "before"]
 	}
 }

implementation.js

@@ -0,0 +1,86 @@
+'use strict';
+
+var isES5 = typeof Object.defineProperty === 'function'
+	&& typeof Object.defineProperties === 'function'
+	&& typeof Object.getPrototypeOf === 'function'
+	&& typeof Object.setPrototypeOf === 'function';
+
+if (!isES5) {
+	throw new TypeError('util.promisify requires a true ES5 environment');
+}
+
+var getOwnPropertyDescriptors = require('object.getownpropertydescriptors');
+
+if (typeof Promise !== 'function') {
+	throw new TypeError('`Promise` must be globally available for util.promisify to work.');
+}
+
+var slice = Function.call.bind(Array.prototype.slice);
+var concat = Function.call.bind(Array.prototype.concat);
+var forEach = Function.call.bind(Array.prototype.forEach);
+
+var hasSymbols = typeof Symbol === 'function' && typeof Symbol('') === 'symbol';
+
+var kCustomPromisifiedSymbol = hasSymbols ? Symbol('util.promisify.custom') : null;
+var kCustomPromisifyArgsSymbol = hasSymbols ? Symbol('customPromisifyArgs') : null;
+
+module.exports = function promisify(orig) {
+	if (typeof orig !== 'function') {
+		var error = new TypeError('The "original" argument must be of type function');
+		error.name = 'TypeError [ERR_INVALID_ARG_TYPE]';
+		error.code = 'ERR_INVALID_ARG_TYPE';
+		throw error;
+	}
+
+	if (hasSymbols && orig[kCustomPromisifiedSymbol]) {
+		var customFunction = orig[kCustomPromisifiedSymbol];
+		if (typeof customFunction !== 'function') {
+			throw new TypeError('The [util.promisify.custom] property must be a function');
+		}
+		Object.defineProperty(customFunction, kCustomPromisifiedSymbol, {
+			configurable: true,
+			enumerable: false,
+			value: customFunction,
+			writable: false
+		});
+		return customFunction;
+	}
+
+	// Names to create an object from in case the callback receives multiple
+	// arguments, e.g. ['stdout', 'stderr'] for child_process.exec.
+	var argumentNames = orig[kCustomPromisifyArgsSymbol];
+
+	var promisified = function fn() {
+		var args = slice(arguments);
+		var self = this; // eslint-disable-line no-invalid-this
+		return new Promise(function (resolve, reject) {
+			orig.apply(self, concat(args, function (err) {
+				var values = arguments.length > 1 ? slice(arguments, 1) : [];
+				if (err) {
+					reject(err);
+				} else if (typeof argumentNames !== 'undefined' && values.length > 1) {
+					var obj = {};
+					forEach(argumentNames, function (name, index) {
+						obj[name] = values[index];
+					});
+					resolve(obj);
+				} else {
+					resolve(values[0]);
+				}
+			}));
+		});
+	};
+
+	Object.setPrototypeOf(promisified, Object.getPrototypeOf(orig));
+
+	Object.defineProperty(promisified, kCustomPromisifiedSymbol, {
+		configurable: true,
+		enumerable: false,
+		value: promisified,
+		writable: false
+	});
+	return Object.defineProperties(promisified, getOwnPropertyDescriptors(orig));
+};
+
+module.exports.custom = kCustomPromisifiedSymbol;
+module.exports.customPromisifyArgs = kCustomPromisifyArgsSymbol;

index.js

@@ -0,0 +1,24 @@
+'use strict';
+
+var define = require('define-properties');
+var util = require('util');
+
+var implementation = require('./implementation');
+var getPolyfill = require('./polyfill');
+var polyfill = getPolyfill();
+var shim = require('./shim');
+
+/* eslint-disable no-unused-vars */
+var boundPromisify = function promisify(orig) {
+/* eslint-enable no-unused-vars */
+	return polyfill.apply(util, arguments);
+};
+define(boundPromisify, {
+	custom: polyfill.custom,
+	customPromisifyArgs: polyfill.customPromisifyArgs,
+	getPolyfill: getPolyfill,
+	implementation: implementation,
+	shim: shim
+});
+
+module.exports = boundPromisify;

package.json

@@ -3,7 +3,10 @@
 	"version": "0.0.0",
 	"description": "Polyfill/shim for util.promisify in node versions < v8",
 	"main": "index.js",
-	"dependencies": {},
+	"dependencies": {
+		"define-properties": "^1.1.2",
+		"object.getownpropertydescriptors": "^2.0.3"
+	},
 	"devDependencies": {
 		"@es-shims/api": "^1.2.0",
 		"@ljharb/eslint-config": "^11.0.0",
@@ -14,7 +17,7 @@
 		"prepublish": "safe-publish-latest",
 		"lint": "eslint .",
 		"pretest": "npm run lint",
-		"tests-only": "es-shim-api",
+		"tests-only": "es-shim-api --bound",
 		"test": "npm run tests-only"
 	},
 	"repository": {

polyfill.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var util = require('util');
+var implementation = require('./implementation');
+
+module.exports = function getPolyfill() {
+	if (typeof util.promisify === 'function') {
+		return util.promisify;
+	}
+	return implementation;
+};

shim.js

@@ -0,0 +1,13 @@
+'use strict';
+
+var util = require('util');
+var getPolyfill = require('./polyfill');
+
+module.exports = function shimUtilPromisify() {
+	var polyfill = getPolyfill();
+	if (polyfill !== util.promisify) {
+		util.promisify = polyfill;
+		Object.defineProperty(util, 'promisify', { value: polyfill });
+	}
+	return polyfill;
+};