A JavaScript object groups related values under named properties. Use an object literal for a one-off record; use a constructor function or a class when you want to create multiple similar objects. In both constructor patterns, new creates an instance, and a prototype lets instances share methods.
What is a JavaScript object?
An object is a collection of properties. Each property pairs a key with a value, and a value can be data or a function. A function stored on an object is commonly called a method. See MDN’s guide to working with objects.
const person = {
name: "Mina",
introduce() {
return `Hi, I'm ${this.name}.`;
},
};
console.log(person.name); // "Mina"
console.log(person.introduce()); // "Hi, I'm Mina."
The braces create an object literal. Use dot notation, as in person.name, to read a property, and call a method with parentheses. Here, this.name refers to the name property on the object receiving the method call.
When should you use a constructor?
An object literal is straightforward when you need one object. If you need several objects with the same kinds of properties and behavior, a constructor function or class gives you a reusable way to initialize them. Each created object is an instance.
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Constructor function
A constructor function is an ordinary function used with new. By convention, its name starts with a capital letter. Assigning values to this inside it creates properties on the new instance.
function Person(name) {
this.name = name;
}
const mina = new Person("Mina");
const dev = new Person("Dev");
console.log(mina.name); // "Mina"
console.log(dev.name); // "Dev"
When new Person("Mina") runs, JavaScript creates an object, calls Person with that object as this, and returns the new object. The function’s job here is to give each instance its own name.
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Class and its constructor method
A class provides another syntax for defining instances. Its constructor() method runs when you create an instance with new and initializes that instance’s properties.
class Person {
constructor(name) {
this.name = name;
}
}
const mina = new Person("Mina");
console.log(mina.name); // "Mina"
The class form and the function-constructor pattern both initialize instances, but they use different syntax. MDN describes the class constructor() as the special method for creating and initializing a class instance; see MDN’s constructor reference.
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How does a prototype make methods available?
Each instance has a prototype link. When JavaScript cannot find a property directly on the instance, it checks the instance’s prototype and continues along the prototype chain. A method placed on a constructor’s prototype can therefore be shared by instances instead of being created separately for each one.
function Person(name) {
this.name = name; // own property: each instance has its own value
}
Person.prototype.introduce = function () {
return `Hi, I'm ${this.name}.`;
};
const mina = new Person("Mina");
const dev = new Person("Dev");
console.log(mina.introduce()); // "Hi, I'm Mina."
console.log(dev.introduce()); // "Hi, I'm Dev."
console.log(Object.hasOwn(mina, "name")); // true
console.log(Object.hasOwn(mina, "introduce")); // false
name is an own property because the constructor assigns it to this. introduce is found through the prototype link, so both instances can use the same method. Methods defined in a class body are also placed on the class’s prototype:
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class Person {
constructor(name) {
this.name = name;
}
introduce() {
return `Hi, I'm ${this.name}.`;
}
}
How do the object-creation options compare?
| Approach | Good fit | Basic syntax | Where shared behavior goes | Requires a constructor definition? |
|---|---|---|---|---|
| Object literal | One-off object | const item = { name: "Mina" }; |
Methods can be properties directly on that object | No |
| Constructor function | Repeated instances using the function-plus-new pattern |
function Person(name) { this.name = name; } |
Person.prototype |
Yes |
| Class | Repeated instances using class syntax | class Person { constructor(name) { this.name = name; } } |
The class prototype; methods go in the class body | Yes, a class definition |
Object.create() |
An object with a deliberately chosen prototype | const person = Object.create(proto); |
The prototype object passed as proto |
No |
Object.create(proto) is an alternative when you want to specify an object’s prototype directly; it does not require a constructor function. MDN covers it alongside object literals and constructor patterns in its object guide.
What does an object’s constructor property mean?
Do not confuse the constructor() method in a class with an object’s constructor property. The class method initializes an instance. The property named constructor is typically inherited from the object’s prototype and often refers to the function that created that prototype.
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That property is not a reliable way to verify an object’s type. It can change if a constructor’s prototype is replaced, and objects created with a null prototype do not inherit the usual Object.prototype properties. MDN documents the inherited property and how prototype reassignment affects it in its Object.prototype.constructor reference. Microsoft Learn notes that most objects inherit from Object.prototype, with null-prototype objects as an exception; see its JavaScript Object reference.
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