Why prototypes are needed in Javascript - Performance Analysis

Time Complexity: Why prototypes are needed

O(n)

Understanding Time Complexity

We want to understand why JavaScript uses prototypes and how this affects the speed of accessing properties and methods.

How does using prototypes change the work the program does when we create many objects?

Scenario Under Consideration

Analyze the time complexity of accessing a method on an object that uses a prototype.

function Person(name) {
  this.name = name;
}
Person.prototype.greet = function() {
  return `Hello, ${this.name}!`;
};

const alice = new Person('Alice');
console.log(alice.greet());
    

This code creates a person object and calls a method defined on its prototype.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Property lookup on the object and its prototype chain.
• How many times: Each method call requires checking the object and possibly its prototype once.

How Execution Grows With Input

When we create more objects, each method call still checks only a small chain of prototypes.

Input Size (n)	Approx. Operations
10	10 property lookups
100	100 property lookups
1000	1000 property lookups

Pattern observation: The work grows linearly with the number of method calls, but each lookup is quick because the prototype chain is short.

Final Time Complexity

Time Complexity: O(n)

This means if you call a method on n objects, the total work grows roughly in a straight line with n.

Common Mistake

[X] Wrong: "Using prototypes makes method calls instant with no work needed."

[OK] Correct: Each method call still needs to check the object and its prototype chain, so some work happens every time.

Interview Connect

Understanding prototypes helps you explain how JavaScript shares methods efficiently and why this matters for performance when many objects exist.

Self-Check

"What if methods were copied directly onto each object instead of using prototypes? How would the time complexity change?"