javascript - how to get an object to return a value that is not the object itself

Question

When creating a x = new Date() object, if you put it into console.log(x) a string will be output.

Is there a way to make a custom object that w

Answer 1

Basically two methods returns values, which are not the object itself.

Object#toString

Every object has a toString() method that is automatically called when the object is to be represented as a text value or when an object is referred to in a manner in which a string is expected. By default, the toString() method is inherited by every object descended from Object. If this method is not overridden in a custom object, `toString() returns "[object type]", where type is the object type.

Object#valueOf

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.

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.

To break it down for your question, you can implement both methods to a custom function.

When both methods are implemented and return primitive values, the valueOf is called first. If valueOf returns an object, toString method is called.

You might have a look to this article: Fake operator overloading in JavaScript

function Custom(value) {
    this.value = value;
}

Custom.prototype.valueOf = function () {
    console.log('valueOf');
    return this.value * 5;
};


var custom = new Custom(7);

console.log('' + custom); // no toString

Custom.prototype.toString = function () {
    console.log('toString');
    return this.value * 3;
};

Custom.prototype.valueOf = function () {
    console.log('valueOf');
    return {};
};

console.log('' + custom);