Add section on "Error Codes" to CONTRIBUTING_JS.md

CONTRIBUTING_JS.md

@@ -19,6 +19,7 @@ Our toolkit for each of these is not set in stone, and we don't plan to halt our
   - [Guidelines](#guidelines)
     - [Supported versions](#supported-versions)
     - [Linting & Code Style](#linting--code-style)
+    - [Error Codes](#error-codes)
     - [Dependency Versions](#dependency-versions)
     - [Testing](#testing)
     - [Releasing](#releasing)
@@ -100,6 +101,41 @@ However, we've added an extra linting rule: Enforce the use of [strict mode](htt
 
 Using [aegir-lint](#aegir) will help you do this easily; it automatically lints your code.
 
+#### Error Codes
+
+When introducing a new error code that may be useful outside of the current scope, make sure it is exported as a new `Error` type:
+
+```js
+class NotFoundError extends Error {
+  constructor (message) {
+    super(message || 'Resource was not found')
+    this.name = 'NotFoundError'
+    this.code = NotFoundError.code
+  }
+}
+
+NotFoundError.code = 'ERR_NOT_FOUND'
+exports.NotFoundError = NotFoundError
+```
+
+
+This enables others to reuse those definitions and decreases the number of hardcoded values across our codebases.  
+For example:
+
+```js
+const { NotFoundError } = require('some-module')
+
+// throw predefined errors types
+if (!value) {
+  throw new NotFoundError()
+}
+
+// compare against code from imported type
+if (err.code === NotFoundError.code) {
+  // handle
+}
+```
+
 #### Dependency Versions
 
 Our rule is: Use ~ for everything below 1.0.0 and ^ for everything above 1.0.0. If you find a package.json that is not following this rule, please submit a PR.