init

.editorconfig

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+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+indent_size = 2
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false

.gitattributes

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+* text=auto

.gitignore

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+coverage
+node_modules

.travis.yml

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+sudo: false
+git:
+  depth: 2
+language: node_js
+node_js:
+  - iojs
+  - node
+after_script:
+  - npm run-script coveralls
+notifications:
+  email: false

LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2015 Shinnosuke Watanabe
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# modify-error-event
+
+[![NPM version](https://img.shields.io/npm/v/modify-error-event.svg)](https://www.npmjs.com/package/modify-error-event)
+[![Build Status](https://travis-ci.org/shinnn/modify-error-event.svg?branch=master)](https://travis-ci.org/shinnn/modify-error-event)
+[![Build status](https://ci.appveyor.com/api/projects/status/s8nn7eflwuqml299?svg=true)](https://ci.appveyor.com/project/ShinnosukeWatanabe/modify-error-event)
+[![Coverage Status](https://img.shields.io/coveralls/shinnn/modify-error-event.svg)](https://coveralls.io/r/shinnn/modify-error-event)
+[![Dependency Status](https://img.shields.io/david/shinnn/modify-error-event.svg?label=deps)](https://david-dm.org/shinnn/modify-error-event)
+[![devDependency Status](https://img.shields.io/david/dev/shinnn/modify-error-event.svg?label=devDeps)](https://david-dm.org/shinnn/modify-error-event#info=devDependencies)
+
+Modify the value of the specific object's `error` [event](https://nodejs.org/api/events.html)
+
+```javascript
+var EventEmitter = require('events').EventEmitter;
+var modifyErrorEvent = require('modify-error-event');
+
+var emitter = new EventEmitter();
+
+modifyErrorEvent(emitter, function(err) {
+  err.message = 'bar';
+  return err;
+});
+
+emitter.on('error', function(err) {
+  err.message; //=> 'bar'
+});
+
+emitter.emit('error', new Error('foo'));
+```
+
+## Installation
+
+[Use npm.](https://docs.npmjs.com/cli/install)
+
+```
+npm install modify-error-event
+```
+
+## API
+
+```javascript
+var modifyErrorEvent = require('modify-error-event');
+```
+
+### modifyErrorEvent(*eventEmitter*, *modifier*)
+
+*eventEmitter*: `Object` (an instance of [`EventEmitter`](https://nodejs.org/api/events.html#events_class_events_eventemitter) or its inheritance e.g. [`Stream`](https://nodejs.org/api/stream.html#stream_stream))  
+*modifier*: `Function`  
+Return: `Object` (Same as the first argument)
+
+It changes the first argument of the `error` event listeners in response to the return value of the *modifier* function.
+
+```javascript
+var EventEmitter = require('events').EventEmitter;
+var modifyErrorEvent = require('modify-error-event');
+
+var emitter = new EventEmitter();
+
+modifyErrorEvent(emitter, function(err) {
+  err.message += 'b';
+  return err;
+});
+
+modifyErrorEvent(emitter, function(val) {
+  err.message += 'c';
+  return err;
+});
+
+emitter
+.on('error', listener)
+.emit('error', new Error('a'));
+
+function listener(err) {
+  err.message; //=> 'abc'
+}
+```
+
+## License
+
+Copyright (c) 2015 [Shinnosuke Watanabe](https://github.com/shinnn)
+
+Licensed under [the MIT License](./LICENSE).

appveyor.yml

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+init:
+  - git config --global core.autocrlf input
+
+shallow_clone: true
+
+version: '{build}'
+
+skip_tags: true
+
+environment:
+  matrix:
+    - nodejs_version: '1'
+    - nodejs_version: '0'
+
+install:
+  - ps: Install-Product node $env:nodejs_version
+  - npm install --production
+  - npm install tape
+
+build: off
+
+test_script:
+  - ps: 'node test.js #PowerShell'
+  - cmd: node test.js
+
+cache:
+  - node_modules

index.js

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+/*!
+ * modify-error-event | MIT (c) Shinnosuke Watanabe
+ * https://github.com/shinnn/modify-error-event
+*/
+'use strict';
+
+var modifyEvent = require('modify-event');
+
+module.exports = function modifyErrorEvent(eventEmitter, fn) {
+  return modifyEvent(eventEmitter, 'error', fn);
+};

package.json

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+{
+  "name": "modify-error-event",
+  "version": "1.0.0",
+  "description": "Modify the value of the specific object's error event",
+  "repository": "shinnn/modify-error-event",
+  "scripts": {
+    "pretest": "eslint --config node_modules/@shinnn/eslintrc-node/rc.json index.js test.js",
+    "test": "node test.js",
+    "coverage": "istanbul cover test.js",
+    "coveralls": "${npm_package_scripts_coverage} && istanbul-coveralls"
+  },
+  "licenses": [
+    {
+      "type": "MIT",
+      "url": "https://github.com/shinnn/modify-error-event/blob/master/LICENSE"
+    }
+  ],
+  "files": [
+    "index.js"
+  ],
+  "keywords": [
+    "modify",
+    "value",
+    "argument",
+    "event",
+    "emit",
+    "error",
+    "change",
+    "update"
+  ],
+  "dependencies": {
+    "modify-event": "^1.0.0"
+  },
+  "devDependencies": {
+    "@shinnn/eslintrc-node": "^1.0.1",
+    "eslint": "^0.20.0",
+    "istanbul": "^0.3.13",
+    "istanbul-coveralls": "^1.0.2",
+    "tape": "^4.0.0"
+  }
+}

test.js

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+'use strict';
+
+var EventEmitter = require('events').EventEmitter;
+
+var modifyErrorEvent = require('./');
+var test = require('tape');
+
+var emitter = new EventEmitter();
+
+test('modifyErrorEvent()', function(t) {
+  t.plan(7);
+
+  t.equal(modifyErrorEvent.name, 'modifyErrorEvent', 'should have a function name.');
+
+  t.strictEqual(
+    modifyErrorEvent(emitter, function(data) {
+      return data * 2;
+    }),
+    emitter,
+    'should return the modified EventEmitter.'
+  );
+
+  emitter
+  .on('error', function(data) {
+    t.strictEqual(this, emitter, 'should call the listener in the same context as original\'s.');
+    t.strictEqual(data, 2, 'should modify the value of event.');
+  })
+  .emit('error', 1);
+
+  t.throws(
+    modifyErrorEvent.bind(null, null, t.fail),
+    /TypeError.* must be an instance of EventEmitter or its inheritance\./,
+    'should throw a type error when the first argument is not an object.'
+  );
+
+  t.throws(
+    modifyErrorEvent.bind(null, {}, t.fail),
+    /TypeError.* must be an instance of EventEmitter or its inheritance\./,
+    'should throw a type error when the first argument is not an instance of EventEmitter.'
+  );
+
+  t.throws(
+    modifyErrorEvent.bind(null, emitter, 'foo'),
+    /TypeError.*foo is not a function. The third argument to modify-event must be a function\./,
+    'should throw a type error when the third argument is not a function.'
+  );
+});