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Object Properties

4 mins

  • Name
    Juleshwar Babu

Table Of Contents

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  • There are two kinds of object properties.
    • The first kind is data properties. All properties that we’ve been using until now were data properties.
      let obj = {
          foo: 10
    // 10 -> data property
    • The second type is accessor property. They are essentially functions that execute on getting and setting a value, but look like regular properties to an external code.

Property Flags and Descriptors

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  • Object properties, besides a value, have three special properties so-called “flags”
    • writable – if true, the value can be changed, otherwise it’s read-only.
    • enumerable – if true, then listed in loops, otherwise not listed.
    • configurable – if true, the property can be deleted and these attributes can be modified, otherwise not.
  • Object.getOwnPropertyDescriptor(obj, propName) allows us to see these flags which are generally hidden away. All these three flags are true for props we normally set.
  • Using Object.defineProperty we can edit these flags for properties. Any flag not mentioned while defining a property is considered false by default
  • Making a property non-configurable is a one-way road. We cannot change it back with defineProperty. The only other change we can make to such a property is make its writable flag from true to false
  • Other handy property configurators
    • Object.preventExtensions(obj): Forbids the addition of new properties to the object.
    • Object.seal(obj): Forbids adding/removing of properties. Sets configurable: false for all existing properties.
    • Object.freeze(obj): Forbids adding/removing/changing of properties. Sets configurable: false, writable: false for all existing properties.

Property Getters and Setters

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    let obj = {
        get propName() {
            // getter, the code executed on getting obj.propName
        set propName(value) {
            // setter, the code executed on setting obj.propName = value
  • Descriptors for accessor properties are different from those for data properties.

    • get – a function without arguments, that works when a property is read,
    • set – a function with one argument, that is called when the property is set,
    • enumerable – same as for data properties… if false, property doesn’t show up in for loops
    • configurable – same as for data properties... if true, the property can be deleted and these descriptors can be modified, otherwise not.
  • A property can either have a value descriptor or get/set descriptors. Not both at the same time.

Content Updates

  • 2023-09-08 - init