feat: error/is-error

README.md

@@ -185,6 +185,19 @@ ensureRegExp(/foo/); // /foo/
 ensureRegExp("foo"); // Thrown TypeError: null is not a regular expression object
 ```
 
+#### Error
+
+The JavaScript _Error_ instance
+
+##### `error/is`
+
+```javascript
+const isError = require("type/error/is");
+
+isError(new Error()); // true
+isError({ mesage: "Fake error" }); // false
+```
+
 ### Tests
 
     $ npm test

error/is.js

@@ -0,0 +1,19 @@
+"use strict";
+
+var isPrototype = require("../prototype/is");
+
+var objectToString = Object.prototype.toString
+  , objectTaggedString = objectToString.call(new Error());
+
+module.exports = function (value) {
+	if (!value) return false;
+
+	// Sanity check (reject objects which do not expose common Error interface)
+	if (typeof value.message !== "string") return false;
+
+	// Ensure its native Error object (has [[ErrorData]] slot)
+	// Note: it's not 100% precise as string tag may be overriden
+	if (objectToString.call(value) !== objectTaggedString) return false;
+
+	return !isPrototype(value);
+};

test/error/is.js

@@ -0,0 +1,42 @@
+"use strict";
+
+var assert  = require("chai").assert
+  , isError = require("../../error/is");
+
+describe("error/is", function () {
+	it("Should return true on error", function () { assert.equal(isError(new Error()), true); });
+
+	it("Should return false on native error with no common API exposed", function () {
+		var value = new Error();
+		value.message = null;
+		assert.equal(isError(value), false);
+	});
+	it("Should return false on Error.prototype", function () {
+		assert.equal(isError(Error.prototype), false);
+	});
+
+	it("Should return false on plain object", function () { assert.equal(isError({}), false); });
+	it("Should return false on function", function () {
+		assert.equal(isError(function () { return true; }), false);
+	});
+
+	it("Should return false on array", function () { assert.equal(isError([]), false); });
+	if (typeof Object.create === "function") {
+		it("Should return false on object with no prototype", function () {
+			assert.equal(isError(Object.create(null)), false);
+		});
+	}
+	it("Should return false on string", function () { assert.equal(isError("foo"), false); });
+	it("Should return false on empty string", function () { assert.equal(isError(""), false); });
+	it("Should return false on number", function () { assert.equal(isError(123), false); });
+	it("Should return false on NaN", function () { assert.equal(isError(NaN), false); });
+	it("Should return false on boolean", function () { assert.equal(isError(true), false); });
+	if (typeof Symbol === "function") {
+		it("Should return false on symbol", function () {
+			assert.equal(isError(Symbol("foo")), false);
+		});
+	}
+
+	it("Should return false on null", function () { assert.equal(isError(null), false); });
+	it("Should return false on undefined", function () { assert.equal(isError(void 0), false); });
+});