Object

Baseline *

20157

Object JavaScript Object Object() /

JavaScript Object Object.prototype Object.prototype null null

Object.prototype Object.prototype JavaScript Object.prototype null

Object prototype

Object.prototype Object.prototype null JavaScript

Object.prototype Object.prototype call()

const obj = {
  foo: 1,
  // 
  // 
  // 
  propertyIsEnumerable() {
    return false;
  },
};

obj.propertyIsEnumerable("foo"); // false; 
Object.prototype.propertyIsEnumerable.call(obj, "foo"); // true;

(Map.prototype.delete() ) delete

null

JavaScript Object.prototype null Object.create(null) __proto__: null : __proto__ Object.prototype.__proto__ null Object.setPrototypeOf(obj, null)

const obj = Object.create(null);
const obj2 = { __proto__: null };

null Object.prototype /

Object.prototype.toString()

const normalObj = {}; // 
const nullProtoObj = Object.create(null); //  "null" 

console.log(`normalObj is: ${normalObj}`); // "normalObj is: [object Object]" 
console.log(`nullProtoObj is: ${nullProtoObj}`); // : Cannot convert object to primitive value 

alert(normalObj); // [object Object] 
alert(nullProtoObj); // : Cannot convert object to primitive value

normalObj.valueOf(); // {} 
nullProtoObj.valueOf(); // : nullProtoObj.valueOf is not a function 

normalObj.hasOwnProperty("p"); // "true" 
nullProtoObj.hasOwnProperty("p"); // : nullProtoObj.hasOwnProperty is not a function 

normalObj.constructor; // "Object() { [native code] }" 
nullProtoObj.constructor; // "undefined"

toString null

nullProtoObj.toString = Object.prototype.toString; //  toString 

console.log(nullProtoObj.toString()); // "[object Object]" 
console.log(`nullProtoObj is: ${nullProtoObj}`); // "nullProtoObj is: [object Object]"

toString() toString() nullProtoObj nullProtoObj (null)

Object.setPrototypeOf(nullProtoObj, Object.prototype) null

null Object.prototype

const ages = { alice: 18, bob: 27 };

function hasPerson(name) {
  return name in ages;
}

function getAge(name) {
  return ages[name];
}

hasPerson("hasOwnProperty"); // true
getAge("toString"); // [Function: toString]

null hasPerson getAge

const ages = Object.create(null, {
  alice: { value: 18, enumerable: true },
  bob: { value: 27, enumerable: true },
});

hasPerson("hasOwnProperty"); // false
getAge("toString"); // undefined

Object.prototype Object.prototype null

const user = {};

// 
Object.prototype.authenticated = true;

// 
if (user.authenticated) {
  // 
}

JavaScript null API

Object.groupBy()
RegExp.prototype.exec() groups indices.groups
Array.prototype[Symbol.unscopables] [Symbol.unscopables] null
import.meta
import * as ns from "module"; import()

null Object.prototype extends null

undefined null TypeError

JavaScript 2

Object.prototype.valueOf(): Object.prototype.valueOf.call(x) x
Object() : Object(x) x undefined null TypeError

for...in object
Array this
Object Object.keys()
this null undefined

toString valueOf

Object(): Object

Object.assign(): 1
Object.create()
Object.defineProperties()
Object.defineProperty()
Object.entries(): [key, value]
Object.freeze()
Object.fromEntries(): [key, value] ( Object.entries )
Object.getOwnPropertyDescriptor()
Object.getOwnPropertyDescriptors()
Object.getOwnPropertyNames()
Object.getOwnPropertySymbols()
Object.getPrototypeOf(): ( [[Prototype]] )
Object.groupBy()
Object.hasOwn(): true false
Object.is(): 2 NaN IsLooselyEqual (==) IsStrictlyEqual (===)
Object.isExtensible()
Object.isFrozen()
Object.isSealed()
Object.keys()
Object.preventExtensions()
Object.seal()
Object.setPrototypeOf(): ( [[Prototype]] )
Object.values()

Object.prototype Object

Object.prototype.__proto__
Object.prototype.constructor: Object Object Constructor.prototype constructor

Object.prototype.__defineGetter__()
Object.prototype.__defineSetter__()
Object.prototype.__lookupGetter__()
Object.prototype.__lookupSetter__()
Object.prototype.hasOwnProperty()
Object.prototype.isPrototypeOf()
Object.prototype.propertyIsEnumerable()
Object.prototype.toLocaleString(): toString()
Object.prototype.toString()
Object.prototype.valueOf()

new

const o1 = new Object();
const o2 = new Object(undefined);
const o3 = new Object(null);

Object()

Object()

o1 o2

// const o1 = new Boolean(true) 
const o1 = new Object(true);

// BigInt() 
// 
const o2 = new Object(1n);

Object.prototype ()

apply() this () Node.prototype Function.prototype

const current = Object.prototype.valueOf;

//  "-prop-value" 
//  Object.prototype 
Object.prototype.valueOf = function (...args) {
  if (Object.hasOwn(this, "-prop-value")) {
    return this["-prop-value"];
  }
  // 
  // 
  // apply  "super" 
  // valueOf() 
  return current.apply(this, args);
};

: prototype


ECMAScript 2027 LanguageSpecification # sec-object-objects

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MDN 2026523

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