With great prototype power comes great responsibility
#javascript#prototype#inheritance#object
Eugene Karataev

Eugene Karataev @karataev

About: undefined is not a function

Location:
Russia, Novosibirsk
Joined:
Apr 15, 2019

With great prototype power comes great responsibility

Publish Date: Jul 13 '19
6 3

Let's say we have a simple object in JavaScript.

let obj = {a: 1};
console.log(obj.a);
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Accessing an object's property have O(1) complexity, i.e. it's super fast. But it's not that simple. If a property is not found, it's searched in the object's prototype. If it's not found again, the process continues until the prototype chain root is reached.

So, the property lookup process might be very time consuming in case of an object with long prototype chain.

Here's an example

function getObjectWithLongPrototypeChain(depth = 1) {
  let nextObj = {a: 1};

  for (let i = 0; i < depth; i++) {
    nextObj = Object.create(nextObj);
  }
  return nextObj;
}

let regularObject = {a: 1};
let longObject = getObjectWithLongPrototypeChain(1000000);

let t0;
let t1;
t0 = performance.now();
console.log(regularObject.a);
t1 = performance.now();
console.log(`Property lookup in the regular object took ${(t1 - t0)} milliseconds.`);

t0 = performance.now();
console.log(longObject.a);
t1 = performance.now();
console.log(`Property lookup in the object with long prototype chain took ${(t1 - t0)} milliseconds.`);
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And the results are

Property lookup in the regular object took 0.21500000730156898 milliseconds.
Property lookup in the object with long prototype chain took 42.249999998603016 milliseconds.
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So, the basic operation of property lookup for the second object took an incredible amount of time! While it's almost impossible to have objects with so long prototype chains in production code, I think it's good to know how JS engine resolves property lookup for objects.

Comments 3 total

  • Anastasia
    AnastasiaJul 13, 2019

    It's a great example of How JS works. Of course, nobody uses somethings as this longObject

    But it makes sense to remember how it dangerous.

  • Budy
    BudyOct 26, 2019

    let Object.prototype.hasOwnProperty() to the rescue

    if (obj.hasOwnProperty('a')) {

}
    • Eugene Karataev
      Eugene KarataevOct 26, 2019

      Yeah, thanks for mentioning this method. But using hasOwnProperty will console.log different result in the post's example, so use it with caution.

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