Object instances share sub-properties?

Question

Given the following code:

var Car = function() {};

Car.prototype = {
  wheels: {
    rims: 'steel'
  }
}

var volvo = new Car;
var mercedes = new Car;

volvo.wheels.rims = 'aluminium';

console.log(volvo.wheels.rims, mercedes.wheels.rims); // 'aluminium', 'aluminium'

Can you explain why instance mercedes of Auto automatically inherits the sub-property definition for rims from volvo?

Note the following code works as expected with same setup:

volvo.wheels = 4;

console.log(volvo.wheels, mercedes.wheels); // 4, Object { rims: 'steel' }

Every instance of `car` shares *the same* object `wheels`. Instance specific properties should be created inside the constructor. — Felix Kling, Oct 08 '14 at 15:52
Maybe the following can help in understanding prototype, constructor and mutating versus assigning members (in combination with prototype). http://stackoverflow.com/questions/16063394/prototypical-inheritance-writing-up/16063711#16063711 — HMR, Oct 09 '14 at 02:45

score 2 · Accepted Answer · edited May 11 '16 at 12:05

2

You only ever create a single object for wheels.

You assign this object to the prototype, so every instance inherits its value.

Javascript will never automatically copy an object.

Instead, you should create the object in the constructor so that you get a new object for each instance.

edited May 11 '16 at 12:05

Simon

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answered Oct 08 '14 at 15:53

SLaks

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Object instances share sub-properties?

1 Answers1