Detailed explanation of how to get and set property attributes in JavaScript

The property of an object in JavaScript has three attributes:

1.writable. Whether the property is writable.
2.enumerable. Whether the property will be enumerated when using the for/in statement.
3. configurable. Whether the properties of this property can be modified and whether the property can be deleted.

In the ECMAScript 3 standard, the values ​​of the above three properties are all true and cannot be changed: the property of the newly created object is writable, enumerable, and deletable; in the ECMAScript 5 standard , these properties can be configured and modified through the property description object (property descriptor).

If the value information of the property is also viewed as an attribute of the property, the property in the object has four attributes: value, writable, enumerable and configurable.

For a property defined with getter and setter methods, since it does not have a writable attribute (whether the property is writable depends on whether the setter method exists), this kind of property also has four attributes: get, set, enumerable and configurable — The value of the get and set properties is function.

Get the properties of the object property

In the ECMAScript 5 standard, you can get the property information of a property of the object itself through Object.getOwnPropertyDescriptor():

var o = {x:1};
var a = Object.create(o);
a.y = 3;
console.log(Object.getOwnPropertyDescriptor(a, "y"));//Object {configurable=true, enumerable=true, writable=true, value=3}
console.log(Object.getOwnPropertyDescriptor(a, "x"));//undefined

As you can see, if the property does not exist or the property is inherited from the prototype object, undefined is returned.

Set the properties of the object property

In the ECMAScript 5 standard, you can set the property of a property of the object itself through Object.defineProperty():

Object.defineProperty(a, "y", {
    value:3,
    writable:true,
    enumerable:false,
    configuration:true
});
console.log(a.propertyIsEnumerable("y"));//false

If the set property is inherited from the prototype object, then JavaScript will create a property with the same name in the object itself, which is consistent with the related behavior of the assignment operation:

Object.defineProperty(a, "x", {
    value:1,
    writable:true,
    enumerable:false,
    configuration:true
});
console.log(a.propertyIsEnumerable("x"));//false
console.log(o.propertyIsEnumerable("x"));//true

In addition to modifying the properties of the property , you can also change the property to be accessed by getter or setter:

Object.defineProperty(a, "y", {
    get:function(){return 42;}
});
console.log(a.y);//42

When using Object.defineProperty(), the property value in the property description object can be partially ignored. When the property value is ignored, in JavaScript The processing rules are as follows:

If the property is newly created, all ignored property values ​​will be false or undefined.
If the property already exists, all ignored property values ​​remain unchanged.

Set the properties of object properties in batches

If you need to set the properties of multiple properties at once, you can use the Object.defineProperties() statement. This statement will return the modified object.

Object.defineProperties(a, {
    "y":{value:79, writable:true, enumerable:true, configurable:true},
    "z":{value:99, writable:true, enumerable:true, configurable:true}
});
console.log(a);//Object {y=79, z=99}

property attribute setting rules

When modifying the property attribute, the following rules must be followed. If the rules are violated, JavaScript will report a TypeError error:

If the object is not extensible, you can only modify the properties of existing properties and cannot add new properties.
If the configurable attribute of the property is false, the values ​​of the configurable and enumerable attributes cannot be modified. For the writable attribute, you can change it from true to false, but you cannot change it from false to true. If a property is defined by getters and setters, the getter and setter methods cannot be modified.
If the configurable attribute and writable attribute of the property are both false, the property value cannot be changed. If the property's writable attribute is false but its configurable attribute is true, the property value can still be modified.

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