nodejs · jasnell · Oct 24, 2016 · joyeecheung · Feb 7, 2017 · jasnell
diff --git a/doc/guides/using-internal-errors.md b/doc/guides/using-internal-errors.md
@@ -0,0 +1,141 @@
+# Using the internal/errors.js Module
+
+## What is internal/errors.js
+
+The `require('internal/errors')` module is an internal-only module that can be
+used to produce `Error`, `TypeError` and `RangeError` instances that use a
+static, permanent error code and an optionally parameterized message.
+
+The intent of the module is to allow errors provided by Node.js to be assigned a
+permanent identifier. Without a permanent identifier, userland code may need to
+inspect error messages to distinguish one error from another. An unfortunate
+result of that practice is that changes to error messages result in broken code
+in the ecosystem. For that reason, Node.js has considered error message changes
+to be breaking changes. By providing a permanent identifier for a specific
+error, we reduce the need for userland code to inspect error messages.
+
+*Note*: Switching an existing error to use the `internal/errors` module must be
+considered a `semver-major` change. However, once using `internal/errors`,
+changes to `internal/errors` error messages will be handled as `semver-minor`
+or `semver-patch`.
+
+## Using internal/errors.js
+
+The `internal/errors` module exposes three custom `Error` classes that
+are intended to replace existing `Error` objects within the Node.js source.
+
+For instance, an existing `Error` such as:
+
+```js
+  var err = new TypeError('Expected string received ' + type);
+```
+
+Can be replaced by first adding a new error key into the `internal/errors.js`
+file:
+
+```js
+E('FOO', 'Expected string received %s');
+```
+
+Then replacing the existing `new TypeError` in the code:
+
+```js
+  const errors = require('internal/errors');
+  // ...
+  var err = new errors.TypeError('FOO', type);
+```
+
+## Adding new errors
+
+New static error codes are added by modifying the `internal/errors.js` file
+and appending the new error codes to the end using the utility `E()` method.
+
+```js
+E('EXAMPLE_KEY1', 'This is the error value');
+E('EXAMPLE_KEY2', (a, b) => return `${a} ${b}`);
+```
+
+The first argument passed to `E()` is the static identifier. The second
+argument is either a String with optional `util.format()` style replacement
+tags (e.g. `%s`, `%d`), or a function returning a String. The optional
+additional arguments passed to the `errors.message()` function (which is
+used by the `errors.Error`, `errors.TypeError` and `errors.RangeError` classes),
+will be used to format the error message.
+
+## Documenting new errors
+
+Whenever a new static error code is added and used, corresponding documentation
+for the error code should be added to the `doc/api/errors.md` file. This will
+give users a place to go to easily look up the meaning of individual error
+codes.
+
+
+## API
+
+### Class: errors.Error(key[, args...])
+
+* `key` {String} The static error identifier
+* `args...` {Any} Zero or more optional arguments
+
+```js
+const errors = require('internal/errors');
+
+var arg1 = 'foo';
+var arg2 = 'bar';
+const myError = new errors.Error('KEY', arg1, arg2);
+throw myError;
+```
+
+The specific error message for the `myError` instance will depend on the
+associated value of `KEY` (see "Adding new errors").
+
+The `myError` object will have a `code` property equal to the `key` and a
+`name` property equal to `Error[${key}]`.
+
+### Class: errors.TypeError(key[, args...])
+
+* `key` {String} The static error identifier
+* `args...` {Any} Zero or more optional arguments
+
+```js
+const errors = require('internal/errors');
+
+var arg1 = 'foo';
+var arg2 = 'bar';
+const myError = new errors.TypeError('KEY', arg1, arg2);
+throw myError;
+```
+
+The specific error message for the `myError` instance will depend on the
+associated value of `KEY` (see "Adding new errors").
+
+The `myError` object will have a `code` property equal to the `key` and a
+`name` property equal to `TypeError[${key}]`.
+
+### Class: errors.RangeError(key[, args...])
+
+* `key` {String} The static error identifier
+* `args...` {Any} Zero or more optional arguments
+
+```js
+const errors = require('internal/errors');
+
+var arg1 = 'foo';
+var arg2 = 'bar';
+const myError = new errors.RangeError('KEY', arg1, arg2);
+throw myError;
+```
+
+The specific error message for the `myError` instance will depend on the
+associated value of `KEY` (see "Adding new errors").
+
+The `myError` object will have a `code` property equal to the `key` and a
+`name` property equal to `RangeError[${key}]`.
+
+### Method: errors.message(key, args)
+
+* `key` {String} The static error identifier
+* `args` {Array} Zero or more optional arguments passed as an Array
+* Returns: {String}
+
+Returns the formatted error message string for the given `key`.
diff --git a/lib/internal/errors.js b/lib/internal/errors.js
@@ -0,0 +1,88 @@
+'use strict';
+
+// The whole point behind this internal module is to allow Node.js to no
+// longer be forced to treat every error message change as a semver-major
+// change. The NodeError classes here all expose a `code` property whose
+// value statically and permanently identifies the error. While the error
+// message may change, the code should not.
+
+const kCode = Symbol('code');
+const messages = new Map();
+
+var assert, util;
+function lazyAssert() {
+  if (!assert)
+    assert = require('assert');
+  return assert;
+}
+
+function lazyUtil() {
+  if (!util)
+    util = require('util');
+  return util;
+}
+
+function makeNodeError(Base) {
+  return class NodeError extends Base {
+    constructor(key, ...args) {
+      super(message(key, args));
+      this[kCode] = key;
+      Error.captureStackTrace(this, NodeError);
+    }
+
+    get name() {
+      return `${super.name}[${this[kCode]}]`;
+    }
+
+    get code() {
+      return this[kCode];
+    }
+  };
+}
+
+function message(key, args) {
+  const assert = lazyAssert();
+  assert.strictEqual(typeof key, 'string');
+  const util = lazyUtil();
+  const msg = messages.get(key);
+  assert(msg, `An invalid error message key was used: ${key}.`);
+  let fmt = util.format;
+  if (typeof msg === 'function') {
+    fmt = msg;
+  } else {
+    if (args === undefined || args.length === 0)
+      return msg;
+    args.unshift(msg);
+  }
+  return String(fmt.apply(null, args));
+}
+
+// Utility function for registering the error codes. Only used here. Exported
+// *only* to allow for testing.
+function E(sym, val) {
+  messages.set(sym, typeof val === 'function' ? val : String(val));
+}
+
+module.exports = exports = {
+  message,
+  Error: makeNodeError(Error),
+  TypeError: makeNodeError(TypeError),
+  RangeError: makeNodeError(RangeError),
+  E // This is exported only to facilitate testing.
+};
+
+// To declare an error message, use the E(sym, val) function above. The sym
+// must be an upper case string. The val can be either a function or a string.
+// The return value of the function must be a string.
+// Examples:
+// E('EXAMPLE_KEY1', 'This is the error value');
+// E('EXAMPLE_KEY2', (a, b) => return `${a} ${b}`);
+//
+// Once an error code has been assigned, the code itself MUST NOT change and
+// any given error code must never be reused to identify a different error.
+//
+// Any error code added here should also be added to the documentation
+//
+// Note: Please try to keep these in alphabetical order
+E('ERR_ASSERTION', (msg) => msg);
+// Add new errors from here...
diff --git a/node.gyp b/node.gyp
@@ -82,6 +82,7 @@
       'lib/internal/cluster/shared_handle.js',
       'lib/internal/cluster/utils.js',
       'lib/internal/cluster/worker.js',
+      'lib/internal/errors.js',
       'lib/internal/freelist.js',
       'lib/internal/fs.js',
       'lib/internal/linkedlist.js',

diff --git a/test/common.js b/test/common.js
@@ -616,3 +616,18 @@ exports.WPT = {
     assert.fail(undefined, undefined, `Reached unreachable code: ${desc}`);
   }
 };
+
+// Useful for testing expected internal/error objects
+exports.expectsError = function expectsError(code, type, message) {
+  return function(error) {
+    assert.strictEqual(error.code, code);
+    if (type !== undefined)
+      assert(error instanceof type, 'error is not the expected type');
+    if (message !== undefined) {
+      if (!util.isRegExp(message))
+        message = new RegExp(String(message));
+      assert(message.test(error.message), 'error.message does not match');
+    }
+    return true;
+  };
+};
diff --git a/test/parallel/test-internal-errors.js b/test/parallel/test-internal-errors.js
@@ -0,0 +1,116 @@
+// Flags: --expose-internals
+'use strict';
+
+const common = require('../common');
+const errors = require('internal/errors');
+const assert = require('assert');
+
+errors.E('TEST_ERROR_1', 'Error for testing purposes: %s');
+errors.E('TEST_ERROR_2', (a, b) => `${a} ${b}`);
+
+const err1 = new errors.Error('TEST_ERROR_1', 'test');
+const err2 = new errors.TypeError('TEST_ERROR_1', 'test');
+const err3 = new errors.RangeError('TEST_ERROR_1', 'test');
+const err4 = new errors.Error('TEST_ERROR_2', 'abc', 'xyz');
+
+assert(err1 instanceof Error);
+assert.strictEqual(err1.name, 'Error[TEST_ERROR_1]');
+assert.strictEqual(err1.message, 'Error for testing purposes: test');
+assert.strictEqual(err1.code, 'TEST_ERROR_1');
+
+assert(err2 instanceof TypeError);
+assert.strictEqual(err2.name, 'TypeError[TEST_ERROR_1]');
+assert.strictEqual(err2.message, 'Error for testing purposes: test');
+assert.strictEqual(err2.code, 'TEST_ERROR_1');
+
+assert(err3 instanceof RangeError);
+assert.strictEqual(err3.name, 'RangeError[TEST_ERROR_1]');
+assert.strictEqual(err3.message, 'Error for testing purposes: test');
+assert.strictEqual(err3.code, 'TEST_ERROR_1');
+
+assert(err4 instanceof Error);
+assert.strictEqual(err4.name, 'Error[TEST_ERROR_2]');
+assert.strictEqual(err4.message, 'abc xyz');
+assert.strictEqual(err4.code, 'TEST_ERROR_2');
+
+assert.throws(
+  () => new errors.Error('TEST_FOO_KEY'),
+  /^AssertionError: An invalid error message key was used: TEST_FOO_KEY.$/);
+// Calling it twice yields same result (using the key does not create it)
+assert.throws(
+  () => new errors.Error('TEST_FOO_KEY'),
+  /^AssertionError: An invalid error message key was used: TEST_FOO_KEY.$/);
+assert.throws(
+  () => new errors.Error(1),
+  /^AssertionError: 'number' === 'string'$/);
+assert.throws(
+  () => new errors.Error({}),
+  /^AssertionError: 'object' === 'string'$/);
+assert.throws(
+  () => new errors.Error([]),
+  /^AssertionError: 'object' === 'string'$/);
+assert.throws(
+  () => new errors.Error(true),
+  /^AssertionError: 'boolean' === 'string'$/);
+assert.throws(
+  () => new errors.TypeError(1),
+  /^AssertionError: 'number' === 'string'$/);
+assert.throws(
+  () => new errors.TypeError({}),
+  /^AssertionError: 'object' === 'string'$/);
+assert.throws(
+  () => new errors.TypeError([]),
+  /^AssertionError: 'object' === 'string'$/);
+assert.throws(
+  () => new errors.TypeError(true),
+  /^AssertionError: 'boolean' === 'string'$/);
+assert.throws(
+  () => new errors.RangeError(1),
+  /^AssertionError: 'number' === 'string'$/);
+assert.throws(
+  () => new errors.RangeError({}),
+  /^AssertionError: 'object' === 'string'$/);
+assert.throws(
+  () => new errors.RangeError([]),
+  /^AssertionError: 'object' === 'string'$/);
+assert.throws(
+  () => new errors.RangeError(true),
+  /^AssertionError: 'boolean' === 'string'$/);
+
+
+// Tests for common.expectsError
+assert.doesNotThrow(() => {
+  assert.throws(() => {
+    throw new errors.TypeError('TEST_ERROR_1', 'a');
+  }, common.expectsError('TEST_ERROR_1'));
+});
+
+assert.doesNotThrow(() => {
+  assert.throws(() => {
+    throw new errors.TypeError('TEST_ERROR_1', 'a');
+  }, common.expectsError('TEST_ERROR_1', TypeError, /^Error for testing/));
+});
+
+assert.doesNotThrow(() => {
+  assert.throws(() => {
+    throw new errors.TypeError('TEST_ERROR_1', 'a');
+  }, common.expectsError('TEST_ERROR_1', TypeError));
+});
+
+assert.doesNotThrow(() => {
+  assert.throws(() => {
+    throw new errors.TypeError('TEST_ERROR_1', 'a');
+  }, common.expectsError('TEST_ERROR_1', Error));
+});
+
+assert.throws(() => {
+  assert.throws(() => {
+    throw new errors.TypeError('TEST_ERROR_1', 'a');
+  }, common.expectsError('TEST_ERROR_1', RangeError));
+}, /^AssertionError: error is not the expected type/);
+
+assert.throws(() => {
+  assert.throws(() => {
+    throw new errors.TypeError('TEST_ERROR_1', 'a');
+  }, common.expectsError('TEST_ERROR_1', TypeError, /^Error for testing 2/));
+}, /^AssertionError: error.message does not match/);