feat: error/ensure

README.md

@@ -198,6 +198,18 @@ isError(new Error()); // true
 isError({ mesage: "Fake error" }); // false
 ```
 
+##### `error/ensure`
+
+If given argument is an error object, it is returned back. Otherwise `TypeError` is thrown.
+
+```javascript
+const ensureError = require("type/error/ensure");
+
+const someError = new Error("Some error");
+ensureError(someError); // someError
+ensureError({ mesage: "Fake error" }); // Thrown TypeError: [object Object] is not an error object
+```
+
 ### Tests
 
     $ npm test

error/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 an error object", arguments[1]);
+};

test/error/ensure.js

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