Default Values of Property Descriptors in JavaScript

When you create a property on an object using the assignment operator or the object literal format, the JavaScript engine creates a property that is writable, configurable, and enumerable, by default.

    let ali = {};
    ali.age = 19;
    
    let susan = {
        age: 14,
    };
    
    console.log(Object.getOwnPropertyDescriptor(ali, "age"));
    console.log(Object.getOwnPropertyDescriptor(susan, "age"));

This is what we get from the last two lines of the code above:

    {
        value: 19,
        writable: true,
        enumerable: true,
        configurable: true,
    }

    {
        value: 14,
        writable: true,
        enumerable: true,
        configurable: true,
    }

You might think that the same happens with Object.defineProperty and Object.defineProperties, too. But this is not the case. The default value of the writable, enumerable, and configurable attributes, when using these functions, is false. If you need the flag to be set, you have to provide the true value yourself.

Here is an example to illustrate this fact:

    let susan = {};
    Object.defineProperty(susan, "age", {
        value: 14,
    });
    
    console.log(Object.getOwnPropertyDescriptor(susan, "age"));

This is the result from this code:

    {
        value: 14,
        writable: false,
        enumerable: false,
        configurable: false,
    }

As you see, the writable, enumerable, and configurable attributes default to a value of false.

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Related Posts

• Object Creation and Manipulation Functions in JavaScript
• Property Descriptors in JavaScript
• Getting and Setting Property Descriptors in JavaScript (Part 1)
• Getting and Setting Property Descriptors in JavaScript (Part 2)
• Getting and Setting Property Descriptors in JavaScript (Part 3)