The new Operator in JavaScript: What Exactly Does It Do?

By Rahul — Google Frontend Engineer

The 4 Steps of new

When you write new Foo(), JavaScript does exactly four things:

1. Creates a new empty object {}
2. Sets the prototype of that object to Foo.prototype
3. Calls Foo() with this bound to the new object
4. If Foo returns an object, use that. Otherwise return the new object from step 1

Implementing new Yourself

The Return Value Trap

Forgetting new

new with Arrow Functions

Best Practices

• Use ES6 classes instead of constructor functions — they enforce new automatically
• If you must use constructor functions, add the instanceof check
• Capitalize constructor function names (convention that signals "use new")
• Never return objects from constructors unless you have a specific reason

Summary

The new operator creates an object, links its prototype, calls the constructor, and handles the return value. Understanding these 4 steps is fundamental to understanding JavaScript's object model.