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    Returns the primitive value of the specified object

    Method of Object
    Implemented in JavaScript 1.1
    ECMAScript Edition ECMAScript 1st Edition






    JavaScript calls the valueOf method to convert an object to a primitive value. You rarely need to invoke the valueOf method yourself; JavaScript automatically invokes it when encountering an object where a primitive value is expected.

    By default, the valueOf method is inherited by every object descended from Object. Every built-in core object overrides this method to return an appropriate value. If an object has no primitive value, valueOf returns the object itself, which is displayed as:

    [object Object]

    You can use valueOf within your own code to convert a built-in object into a primitive value. When you create a custom object, you can override Object.prototype.valueOf to call a custom method instead of the default Object method.

    Overriding valueOf for custom objects

    You can create a function to be called in place of the default valueOf method. Your function must take no arguments.

    Suppose you have an object type myNumberType and you want to create a valueOf method for it. The following code assigns a user-defined function to the object's valueOf method:

    myNumberType.prototype.valueOf = function () { return customPrimitiveValue; };

    With the preceding code in place, any time an object of type myNumberType is used in a context where it is to be represented as a primitive value, JavaScript automatically calls the function defined in the preceding code.

    An object's valueOf method is usually invoked by JavaScript, but you can invoke it yourself as follows:



    Objects in string contexts convert via the toString method, which is different from String objects converting to string primitives using valueOf. All objects have a string conversion, if only "[object type ]". But many objects do not convert to number, boolean, or function.



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