feat: promise/ensure

README.md

@@ -568,6 +568,18 @@ isPromise({ then: () => {} }); // false
 isPromise({}); // false
 ```
 
+##### `promise/ensure`
+
+If given argument is a promise, it is returned back. Otherwise `TypeError` is thrown.
+
+```javascript
+const ensurePromise = require("type/promise/ensure");
+
+const promise = Promise.resolve();
+ensurePromise(promise); // promise
+eensurePromise({}); // Thrown TypeError: [object Object] is not a promise
+```
+
 ---
 
 #### Thenable

promise/ensure.js

@@ -0,0 +1,9 @@
+"use strict";
+
+var resolveException = require("../lib/resolve-exception")
+  , is               = require("./is");
+
+module.exports = function (value/*, options*/) {
+	if (is(value)) return value;
+	return resolveException(value, "%v is not a promise", arguments[1]);
+};

test/promise/ensure.js

@@ -0,0 +1,20 @@
+"use strict";
+
+var assert        = require("chai").assert
+  , ensurePromise = require("../../promise/ensure");
+
+describe("promise/ensure", function () {
+	it("Should return input value", function () {
+		var value = Promise.resolve();
+		assert.equal(ensurePromise(value), value);
+	});
+	it("Should crash on no value", function () {
+		try {
+			ensurePromise({});
+			throw new Error("Unexpected");
+		} catch (error) {
+			assert.equal(error.name, "TypeError");
+			assert.equal(error.message, "[object Object] is not a promise");
+		}
+	});
+});