feat: function/ensure

README.md

@@ -387,6 +387,18 @@ isFunction(class {}); // true
 isFunction("foo"); // false
 ```
 
+#### `function/ensure`
+
+If given argument is a function object, it is returned back. Otherwise `TypeError` is thrown.
+
+```javascript
+const ensureFunction = require("type/function/ensure");
+
+const fn = function () {};
+ensureDate(fn); // fn
+ensureDate(/foo/); // Thrown TypeError: /foo/ is not a function
+```
+
 ---
 
 ### RegExp

function/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 function", arguments[1]);
+};

test/function/ensure.js

@@ -0,0 +1,20 @@
+"use strict";
+
+var assert         = require("chai").assert
+  , ensureFunction = require("../../function/ensure");
+
+describe("function/ensure", function () {
+	it("Should return input value", function () {
+		var value = function () { return true; };
+		assert.equal(ensureFunction(value), value);
+	});
+	it("Should crash on invalid value", function () {
+		try {
+			ensureFunction(null);
+			throw new Error("Unexpected");
+		} catch (error) {
+			assert.equal(error.name, "TypeError");
+			assert(error.message.includes("is not a function"));
+		}
+	});
+});