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# node-error-ex [](https://travis-ci.org/Qix-/node-error-ex) [](https://coveralls.io/r/Qix-/node-error-ex) > Easily subclass and customize new Error types ## Examples To include in your project: ```javascript var errorEx = require('error-ex'); ``` To create an error message type with a specific name (note, that `ErrorFn.name` will not reflect this): ```javascript var JSONError = errorEx('JSONError'); var err = new JSONError('error'); err.name; //-> JSONError throw err; //-> JSONError: error ``` To add a stack line: ```javascript var JSONError = errorEx('JSONError', {fileName: errorEx.line('in %s')}); var err = new JSONError('error') err.fileName = '/a/b/c/foo.json'; throw err; //-> (line 2)-> in /a/b/c/foo.json ``` To append to the error message: ```javascript var JSONError = errorEx('JSONError', {fileName: errorEx.append('in %s')}); var err = new JSONError('error'); err.fileName = '/a/b/c/foo.json'; throw err; //-> JSONError: error in /a/b/c/foo.json ``` ## API #### `errorEx([name], [properties])` Creates a new ErrorEx error type - `name`: the name of the new type (appears in the error message upon throw; defaults to `Error.name`) - `properties`: if supplied, used as a key/value dictionary of properties to use when building up the stack message. Keys are property names that are looked up on the error message, and then passed to function values. - `line`: if specified and is a function, return value is added as a stack entry (error-ex will indent for you). Passed the property value given the key. - `stack`: if specified and is a function, passed the value of the property using the key, and the raw stack lines as a second argument. Takes no return value (but the stack can be modified directly). - `message`: if specified and is a function, return value is used as new `.message` value upon get. Passed the property value of the property named by key, and the existing message is passed as the second argument as an array of lines (suitable for multi-line messages). Returns a constructor (Function) that can be used just like the regular Error constructor. ```javascript var errorEx = require('error-ex'); var BasicError = errorEx(); var NamedError = errorEx('NamedError'); // -- var AdvancedError = errorEx('AdvancedError', { foo: { line: function (value, stack) { if (value) { return 'bar ' + value; } return null; } } }) var err = new AdvancedError('hello, world'); err.foo = 'baz'; throw err; /* AdvancedError: hello, world bar baz at tryReadme() (readme.js:20:1) */ ``` #### `errorEx.line(str)` Creates a stack line using a delimiter > This is a helper function. It is to be used in lieu of writing a value object > for `properties` values. - `str`: The string to create - Use the delimiter `%s` to specify where in the string the value should go ```javascript var errorEx = require('error-ex'); var FileError = errorEx('FileError', {fileName: errorEx.line('in %s')}); var err = new FileError('problem reading file'); err.fileName = '/a/b/c/d/foo.js'; throw err; /* FileError: problem reading file in /a/b/c/d/foo.js at tryReadme() (readme.js:7:1) */ ``` #### `errorEx.append(str)` Appends to the `error.message` string > This is a helper function. It is to be used in lieu of writing a value object > for `properties` values. - `str`: The string to append - Use the delimiter `%s` to specify where in the string the value should go ```javascript var errorEx = require('error-ex'); var SyntaxError = errorEx('SyntaxError', {fileName: errorEx.append('in %s')}); var err = new SyntaxError('improper indentation'); err.fileName = '/a/b/c/d/foo.js'; throw err; /* SyntaxError: improper indentation in /a/b/c/d/foo.js at tryReadme() (readme.js:7:1) */ ``` ## License Licensed under the [MIT License](http://opensource.org/licenses/MIT). You can find a copy of it in [LICENSE](LICENSE).