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JavaScript finds properties by checking an object first, then following its [[Prototype]] link through a chain until it finds a match or reaches null. Understanding that lookup explains how instances share methods, why an own property can shadow an inherited one, and how class, constructor functions, and Object.create() express the same underlying prototype model.
What a prototype means in JavaScript
Every ordinary JavaScript object has an internal link called [[Prototype]]. The link points to another object, which can have its own prototype link. Together, these links form the prototype chain. When code reads a property, JavaScript checks the object itself first, then each linked prototype in turn. If no object in the chain has that property, lookup ends at null.
This is delegation, not copying: an instance can use a method stored on a shared prototype without having a separate copy of that method as one of its own properties. MDN describes classes as syntax that does not introduce a new inheritance pattern; classes use JavaScript’s prototype-based system (MDN: Inheritance and the prototype chain).
Distinguish [[Prototype]] from .prototype
The similar names refer to different things. [[Prototype]] is an object’s internal link. A function or class’s .prototype is an ordinary property whose object is used as the prototype for instances created with new. The standard inspection function Object.getPrototypeOf(obj) returns the object’s prototype link.
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[[Prototype]]: the internal link on an object that supports inherited lookup.Constructor.prototype: the object that normally becomes the[[Prototype]]of an instance made withnew Constructor().Object.getPrototypeOf(obj): the standard API for inspecting an object’s prototype link.obj.__proto__: a legacy accessor found in JavaScript implementations; preferObject.getPrototypeOf()for inspection. It is different from the standardized object-literal syntax{ __proto__: proto }, which sets the literal’s prototype.
For example, a typical instance created from a constructor has this chain: instance → Constructor.prototype → Object.prototype → null.
Trace a property lookup
A Date instance inherits methods such as getTime() from Date.prototype. The method is available even though it is not an own property of the instance:
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const date = new Date();
Object.getPrototypeOf(date) === Date.prototype; // true
Object.hasOwn(date, "getTime"); // false
date.getTime(); // found on Date.prototypeObject.hasOwn() checks whether a property belongs directly to the object. It is useful when you need to distinguish an own property from one found higher in the chain. For example, a property can exist on an object or its prototype with the value undefined; checking the value alone does not tell you whether the property exists or where it is owned.
Understand shadowing
If an object has its own property with the same name as an inherited property, lookup stops at the nearer own property. This is called shadowing. The inherited property remains on the prototype; the instance’s property takes precedence when accessed through that instance.
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const date = new Date();
date.getTime = () => "custom";
date.getTime(); // "custom"Here the instance’s own getTime function is found before Date.prototype.getTime. The change affects this date object, not every date instance.
Share methods with a constructor function
Constructor functions make the connection between instance state and shared behavior explicit. Put per-instance values on this inside the constructor, and put methods on the constructor’s .prototype when instances should share them:
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function Person(name) {
this.name = name;
}
Person.prototype.greet = function () {
return `Hello, ${this.name}`;
};
const ada = new Person("Ada");
ada.greet(); // "Hello, Ada"With new Person("Ada"), JavaScript creates an instance whose prototype is Person.prototype, then calls the constructor with that instance as this. The instance owns name; the inherited greet method is shared through the prototype.
Use Object.create() to choose a prototype directly
Object.create(proto) creates an object with the supplied object as its prototype, without requiring a constructor function:
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const personPrototype = {
greet() {
return `Hello, ${this.name}`;
},
};
const ada = Object.create(personPrototype);
ada.name = "Ada";
ada.greet(); // "Hello, Ada"This approach is useful when direct delegation is clearer than defining a constructor. You can also call Object.create(null) to make an object with no prototype. Such an object does not inherit methods from Object.prototype, so do not assume methods such as hasOwnProperty are available on it. Use Object.hasOwn(obj, key) to test ownership.
How class and extends fit in
Class syntax offers a familiar way to define constructors and prototype methods. It uses the same prototype inheritance mechanism rather than a separate kind of object inheritance:
class Person {
constructor(name) {
this.name = name;
}
greet() {
return `Hello, ${this.name}`;
}
}
class Developer extends Person {}
const ada = new Developer("Ada");
ada.greet(); // "Hello, Ada"The instance’s greet() lookup follows the class prototype chain: Developer.prototype links to Person.prototype, which links to Object.prototype, then null. The instance’s name is its own property, initialized by the inherited constructor.
Choose the construction style that makes the relationship clear
| Approach | How the prototype relationship is established | When it is useful |
|---|---|---|
class and extends |
Class syntax sets up the prototype relationships. | A readable, familiar choice for many modern examples and codebases. |
Constructor function and new |
The instance receives the constructor’s .prototype as its [[Prototype]]. |
Useful for understanding existing code and seeing how instances share prototype methods. |
Object.create(proto) |
The chosen prototype is supplied directly at object creation. | Useful when direct delegation is desired without a constructor function. |
These options differ mainly in how they express object creation and how familiar they will be to the people maintaining the code. The mechanism is prototype delegation in each case; there is no universally best style or supported performance winner among them.
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Prototype pitfalls to avoid
- Do not use
__proto__as your inspection API. UseObject.getPrototypeOf(obj)instead. - Avoid routine prototype changes after object creation. Establish the intended relationship when creating objects where practical; changing prototypes can affect performance and code behavior.
- Keep inheritance chains understandable. Very long or complicated chains can create maintenance challenges and may cause performance problems; there is no universal chain-length threshold.
- Do not extend built-in prototypes in ordinary application code. MDN advises against it except when supporting compatibility with newer JavaScript features (MDN guidance).
- Be careful when replacing a constructor’s entire
.prototypeobject. A replacement can remove the conventionalconstructorproperty, which can confuse code that relies on it. - Do not infer absence from an
undefinedvalue. Use an ownership or existence check when the distinction matters.
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