feat: Promise.lazy

Constructor for lazy promises

index.js

@@ -16,6 +16,7 @@ module.exports = {
 	math: require("./math"),
 	number: require("./number"),
 	object: require("./object"),
+	promise: require("./promise"),
 	regExp: require("./reg-exp"),
 	string: require("./string")
 };

promise/.eslintrc.json

@@ -0,0 +1,3 @@
+{
+  "globals": { "Promise": true }
+}

promise/index.js

@@ -0,0 +1,5 @@
+"use strict";
+
+module.exports = {
+	lazy: require("./lazy")
+};

promise/lazy.js

@@ -0,0 +1,38 @@
+"use strict";
+
+var isFunction = require("../function/is-function");
+
+module.exports = function (executor) {
+	var Constructor;
+	if (isFunction(this)) {
+		Constructor = this;
+	} else if (typeof Promise === "function") {
+		Constructor = Promise;
+	} else {
+		throw new TypeError("Could not resolve Promise constuctor");
+	}
+
+	var lazyThen;
+	var promise = new Constructor(function (resolve, reject) {
+		lazyThen = function (onSuccess, onFailure) {
+			if (!hasOwnProperty.call(this, "then")) {
+				// Sanity check
+				throw new Error("Unexpected (inherited) lazy then invocation");
+			}
+
+			try {
+				executor(resolve, reject);
+			} catch (reason) {
+				reject(reason);
+			}
+			delete this.then;
+			return this.then(onSuccess, onFailure);
+		};
+	});
+
+	return Object.defineProperty(promise, "then", {
+		configurable: true,
+		writable: true,
+		value: lazyThen
+	});
+};

test/promise/.eslintrc.json

@@ -0,0 +1,3 @@
+{
+  "globals": { "setTimeout": true }
+}

test/promise/lazy.js

@@ -0,0 +1,52 @@
+"use strict";
+
+module.exports = function (t) {
+	if (typeof Promise !== "function") return null; // Run tests only in ES2015+ env
+
+	return {
+		"Delays execution": function (a, d) {
+			var invoked = false;
+			var promise = t(function (resolve) {
+				invoked = true;
+				setTimeout(function () {
+					resolve(20);
+				}, 10);
+			});
+
+			a(invoked, false);
+
+			setTimeout(function () {
+				a(invoked, false);
+				promise.then(function (value) {
+					a(value, 20);
+					setTimeout(d, 0); // Escape error swallowing
+				});
+				a(invoked, true);
+			}, 15);
+		},
+		"Passes rejection": function (a, d) {
+			var promise = t(function (resolve, reject) {
+				setTimeout(function () {
+					reject(new Error("Stop"));
+				}, 10);
+			});
+
+			promise.catch(function (error) {
+				a(error instanceof Error, true);
+				a(error.message, "Stop");
+				setTimeout(d, 0); // Escape error swallowing
+			});
+		},
+		"Passes sync exception": function (a, d) {
+			var promise = t(function () {
+				throw new Error("Stop");
+			});
+
+			promise.catch(function (error) {
+				a(error instanceof Error, true);
+				a(error.message, "Stop");
+				setTimeout(d, 0); // Escape error swallowing
+			});
+		}
+	};
+};