WIP isHttpError

README.md

@@ -88,6 +88,14 @@ fs.readFile('foo.txt', function (err, buf) {
 - `error` - the error object to extend
 - `properties` - custom properties to attach to the object
 
+### createError.isHttpError(val)
+
+Determine if the provided `val` is an `HttpError`. This will return `true`
+if the error inherits from the `HttpError` constructor of this module or
+matches the "duck type" for an error this module creates. All outputs from
+the `createError` factory will return `true` for this function, including
+if an non-`HttpError` was passed into the factory.
+
 ### new createError\[code || name\](\[msg]\))
 
 Create a new error object with the given message `msg`.

index.js

@@ -25,6 +25,7 @@ var toIdentifier = require('toidentifier')
 
 module.exports = createError
 module.exports.HttpError = createHttpErrorConstructor()
+module.exports.isHttpError = createIsHttpErrorFunction(module.exports.HttpError)
 
 // Populate exports for all constructors
 populateConstructorExports(module.exports, statuses.codes, module.exports.HttpError)
@@ -172,6 +173,27 @@ function createClientErrorConstructor (HttpError, name, code) {
   return ClientError
 }
 
+/**
+ * Create function to test is a value is a HttpError.
+ * @private
+ */
+
+function createIsHttpErrorFunction (HttpError) {
+  return function isHttpError (val) {
+    if (!val || typeof val !== 'object') {
+      return false
+    }
+
+    if (val instanceof HttpError) {
+      return true
+    }
+
+    return val instanceof Error &&
+      typeof val.expose === 'boolean' &&
+      typeof val.statusCode === 'number' && val.status === val.statusCode
+  }
+}
+
 /**
  * Create a constructor for a server error.
  * @private

test/test.js

@@ -122,6 +122,62 @@ describe('createError(status, message)', function () {
   })
 })
 
+describe('createError.isHttpError(val)', function () {
+  describe('when val is undefined', function () {
+    it('should return false', function () {
+      assert.strictEqual(createError.isHttpError(undefined), false)
+    })
+  })
+
+  describe('when val is null', function () {
+    it('should return false', function () {
+      assert.strictEqual(createError.isHttpError(null), false)
+    })
+  })
+
+  describe('when val is a number', function () {
+    it('should return false', function () {
+      assert.strictEqual(createError.isHttpError(42), false)
+    })
+  })
+
+  describe('when val is a string', function () {
+    it('should return false', function () {
+      assert.strictEqual(createError.isHttpError('foobar'), false)
+    })
+  })
+
+  describe('when val is an empty object', function () {
+    it('should return false', function () {
+      assert.strictEqual(createError.isHttpError({}), false)
+    })
+  })
+
+  describe('when val is a plain Error', function () {
+    it('should return false', function () {
+      assert.strictEqual(createError.isHttpError(new Error()), false)
+    })
+  })
+
+  describe('when val is an instance of HttpError', function () {
+    it('should return true', function () {
+      var err = createError(500)
+
+      assert.ok(err instanceof createError.HttpError)
+      assert.strictEqual(createError.isHttpError(err), true)
+    })
+  })
+
+  describe('when val is an Error passed to createError', function () {
+    it('should return true', function () {
+      var err = createError(500, new Error())
+
+      assert.ok(!(err instanceof createError.HttpError))
+      assert.strictEqual(createError.isHttpError(err), true)
+    })
+  })
+})
+
 describe('HTTP Errors', function () {
   it('createError(status, props)', function () {
     var err = createError(404, {