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    0.1.5 • Public • Published


    ExtendThis is an extensible framework for defining "recipes" for extending an object using properties from other objects. It evolved from the need to selectively compose an object's API using composition (rather than inheritance). Out-of-the-box, it supports extending objects using delegation and mixins. However, its API has been generalized to allow it to support arbitrary recipes for extending objects.

    Table of Contents


    • An extensible set of selectors for selecting object properties.
    • An extensible set of filters for filtering object properties.
    • An extensible set of methods for applying preselected filters to object properties.
    • Detection of namespace collisions when merging object properties.
    • Ability to rename and redefine the default APIs according to your preferences.
    • Compatible with CommonJS, AMD, and non-module build environments.
    • Less than 5K minified.


    To install the package:

    npm install extend-this

    To require the package:

    var extend = require("extend-this");



    To delegate method calls to another object, use the .withDelegate() method:

     * @constructor
    function Mutant() {
            .withDelegate(new Dog())
            .withDelegate(new Cat());
    var mutant = new Mutant();

    Properties beginning with underscore are ignored, and non-function properties are copied (shallow).


    To mixin the APIs from another object, use the .withCall() and .with() methods:

     * @constructor
    function Rectangle(length, width) {
        this._length = length;
        this._width = width;
    Rectangle.prototype.area = function() {
        return this._length * this._width;
     * @constructor
    function MyShape() {
        extend(this).withCall(Rectangle, 5, 4);
    var myShape = new MyShape();

    Note: If you want to use the .withCall() method in combination with the selectors and filters described below, pass the constructor and its arguments in an array followed by the filter or selector arguments.

    extend(this).withCall([Rectangle, 5, 4], '!type');

    Property Selectors

    By default all properties of the source object are merged with the properties of the target object. However this can be overridden by using property selectors.

    String Selector

    Merges the properties with the specified name:

    extend(this).withDelegate(new Dog(), 'bark', 'owner');

    Negation Selector

    If no properties are currently selected, it merges all properties except the ones prefixed with the '!' character. Otherwise it simply unselects the properties prefixed with the '!' character from the currently selected properties.

    extend(this).withDelegate(new Dog(), '!type', '!color');

    Regular Expression Selector

    Merges all properties whose names match the regular expression:

    extend(this).withDelegate(new Dog(), /bark/);

    Rename Selector

    Merges the properties with the specified names, but renames the properties when applied to the target object:

     * @constructor
    function Mutant() {
        extend(this).withDelegate(new Dog(), {
            bark: sound,
            owner: person
    var mutant = new Mutant();

    Override Selector

    Merges the property with the specified name, and doesn't report an error if the property already exists in the target object:

    extend(this).withDelegate(new Dog(), '#bark');

    Selector Combinations

    Different property selectors can be used together.

    This example merges all properties and renames the bark() method to the sound() method:

    extend(this).withDelegate(new Dog(), /.*/, {bark: sound});

    This example renames the bark() method to the sound() method and doesn't report an error if the sound() method already exists in the target object:

    extend(this).withDelegate(new Dog(), {'#bark': sound});

    Adding a Custom Selector

    To add your own property selector, use the .selector() method. In this example, mySelector will be invoked whenever a string argument is prefixed with '*':

    extend.selector('*', mySelector);

    When a selector is invoked, it is passed a selectorContext object. The selector should update the embedded sourceKeys object with the names of the properties to merge into the target object.

    The following mySelector function merges all the properties from the source object and disables reporting an error if the property already exists in the target object:

     * A selector which merges all properties but doesn't report an error if the
     * property already exists in the target object:
     * @param selectorContext.source {object}
     *     The source object (read-only).
     * @param selectorContext.sourceKey {string}
     *     The remaining text after the selector prefix (read-only).
     * @param selectorContext.targetKey
     *     The key to use when applying the property to the target (read-only).
     * @param selectorContext.sourceKeys {object}
     *     The keys of the object are the keys of the properties to merge into
     *     the target object. The values of the object are the target keys.
     *     (modifiable).
     * @param selectorContext.overrideKeys {object} 
     *     The keys of the object are the source keys which should not report
     *     an error if the target key already exists in the target object (modifiable).
    function mySelector(selectorContext) {
        var source = selectorContext.source;
            sourceKeys = selectorContext.sourceKeys;
            overrideKeys = selectorContext.overrideKeys;
        for (var key in source) {
            sourceKeys[key] = key;
            overrideKeys[key] = true;

    Changing a Selector Prefix

    To change when an existing property selector is invoked, use the .selector() method. The following example causes the override selector to be invoked when a string argument is prefixed with '@' instead of '#':

    extend.selector('@', extend.selector('#'));
    extend.selector('#', null);


    Filters allow you to reject selected properties, and transform the values of selected properties. Filters form a filter pipeline where a property is passed from one filter to the next.

    Adding a Custom Filter

    To add your own filter, simply add it as another argument to the method call:

    extend(this).with(new Dog(), '!bark', myFilter, anotherFilter);

    When a filter is invoked, it is passed a filterContext object which contains the property being merged. A filter is expected to return true if the property can be merged, and false if the property should be rejected. The filter can modify the property's value and the target name.

    Here is the filter used by the .withDelegate() method to delegate method calls to the source object:

     * A filter which transforms the source functions to functions which can
     * be called from the context of the target object.
     * @param {object}
     *    The target object (read-only).
     * @param filterContext.source {object}
     *    The source object (read-only).
     * @param filterContext.sourceKey {string}
     *    The source property key (read-only).
     * @param filterContext.sourceValue {*} 
     *    The source property value (modifiable).
     * @param filterContext.targetKey {string}
     *    The target key (modifiable).
     * @returns {boolean} true to allow property to be merged
    function delegateFilter(filterContext) {
        if (isFunction(filterContext.sourceValue)) {
            var func = filterContext.sourceValue;
            var source = filterContext.source;
            filterContext.sourceValue = function() {
                return fastApply(func, source, arguments);
        return true;

    Here is the filter used by the .withDelegate() method to exclude all properties beginning with the underscore character.

     * Returns a filter which excludes properties that match the
     * provided regexp.
     * @param {RegExp}
     * @returns {function} The filter.
    function createExcludeNameFilter(regexp) {
        return function(filterContext) {
            return !regexp.test(filterContext.sourceKey);

    The following example is equivalent to calling .withDelegate(new Dog()):

    extend(this).with(new Dog(), createExcludeNameFilter(/^_/), delegateFilter);


    Methods allow you to fully operate on the target and source object. However their primary purpose is to allow you to preselect which filters are applied to the properties rather than have the user pass in the filters as arguments.

    Adding a Custom Method

    To add your own method, use .method(). In the following example, delegateMethod is invoked when the user calls .withDelegate():

    extend.method('withDelegate', delegateMethod);

    When a method is invoked, it is passed a parser function and any user arguments. The method is responsible for calling the parser function with the user arguments and returning the parameters from the parser function. But before doing so, a method can:

    • modify the user arguments before passing them to the parser function
    • modify the parameters returned by the parser function

    In this example, the delegateMethod modifies the parameters returned by the parser function by adding the excludeNameFilter at the front of the filter pipeline, and the delegateFilter at the end of the filter pipeline.

     * A method which delegates method calls from the target object to
     * the source object.
     * @param {Object} target The target object.
     * @param {function} parseArgs The parser function.
     * @param {args} args The arguments passed to the method.
     * @returns {Object} params The values from the parsed arguments.
     * @returns {object} params.source The source object.
     * @returns {array} params.filters The filters.
     * @returns {object} params.sourceKeys
     *     The keys of the object are the keys of the properties to merge into
     *     the target object. The values of the object are the target keys.
     * @returns {object} params.overrideKeys
     *     The keys of the object are the source keys which should not report
     *     an error if the target key already exists in the target object.
    function delegateMethod(target, parseArgs, args) {
        var params = parseArgs(args);
        return params;

    Note: One of the advantages of adding filters using a method is that you have absolute control over where the filters are placed in relation to the user arguments.

    Changing a Method Name

    To change when an existing method is invoked, use .method(). The following example will cause the mixin method to be invoked when the user calls .withMixin() instead of .with():

    extend.method('withMixin', extend.method('with'));
    extend.method('with', null);

    Array Arguments

    Arguments passed to a method can be included in one or more arrays. The arrays can be nested. This allows arguments to be packaged as a single argument.

    var myRecipe = [];
    myRecipe.push(['propA', 'propB']);
    extend(this).with(source, myRecipe);


    To turn off namespace collision detection:

    extend.config.throwOverrideError = false;

    To turn off property not found errors:

    extend.config.throwPropertyNotFoundError = false;

    Multiple extend Functions

    There may be another type of extend function which you may want to use in addition to extendThis. To allow the other extend function to be invoked from extendThis, use the .wrap() method:


    The other extend function will be invoked whenever extendThis is passed more than one argument:

    extend(target, source);


    ExtendThis cannot detect namespace collisions if a property is added after its API call. For example:

    extend(this).with(source, 'foo'); = 'red';

    This will not generate an error. Dependending upon the context, you may want to be alerted that the property 'foo' from the source object, collides with the local property 'foo'.

    ExtendThis offers an alternative way of setting the 'foo' property using the following method call:

        .with(source, 'foo')
        .with('foo', 'red');

    In this case, an error will be generated that the 'foo' property already exists.


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