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    Object.prototype.isPrototypeOf()

    Summary

    The isPrototypeOf() method tests for an object in another object's prototype chain.

    Note: isPrototypeOf differs from the instanceof operator. In the expression "object instanceof AFunction", the object prototype chain is checked against AFunction.prototype, not against AFunction itself.

    Syntax

    prototypeObj.isPrototypeOf(obj)

    Parameters

    prototypeObj
    An object to be tested against each link in the prototype chain of the object argument.
    object
    The object whose prototype chain will be searched.

    Description

    The isPrototypeOf method allows you to check whether or not an object exists within another object's prototype chain.

    For example, consider the following prototype chain:

    function Fee() {
      // ...
    }
    
    function Fi() {
      // ...
    }
    Fi.prototype = new Fee();
    
    function Fo() {
      // ...
    }
    Fo.prototype = new Fi();
    
    function Fum() {
      // ...
    }
    Fum.prototype = new Fo();
    

    Later on down the road, if you instantiate Fum and need to check if Fi's prototype exists within the Fum prototype chain, you could do this:

    var fum = new Fum();
    // ...
    
    if (Fi.prototype.isPrototypeOf(fum)) {
      // do something safe
    }
    

    This, along with the instanceof operator particularly comes in handy if you have code that can only function when dealing with objects descended from a specific prototype chain, e.g., to guarantee that certain methods or properties will be present on that object.

    Specifications

    Specification Status Comment
    ECMAScript 3rd Edition. Standard Initial definition.
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Object.prototype.hasOwnProperty' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'Object.prototype.hasOwnProperty' in that specification.
    Draft  

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

    See also

    Document Tags and Contributors

    Last updated by: FredKSchott,