new constructors in javascript object methods

Question

Hey i'm just looking for a javascript explanation to explain why 'constructor called' is only called once as opposed to 5 times?

const Dog = function(name, age){
  this.name = name; 
  this.age = age; 
  console.log('constructor called'); 
}

const obj = { 
  dog: new Dog('roofus', 20)
}

const main = ()=>{
  for(let i = 0; i < 5; i++){
    console.log(obj.dog)
  }
}

main();
'constructor called'
Dog { name: 'roofus', age: 20 }
Dog { name: 'roofus', age: 20 }
Dog { name: 'roofus', age: 20 }
Dog { name: 'roofus', age: 20 }
Dog { name: 'roofus', age: 20 }

why do you think Dog is called 5 times? The instance that you've created is logged 5 times, but new Dog() is only called once, when you define obj. — Robin Zigmond
– Robin Zigmond, Commented Nov 16, 2020 at 22:35
is it because the new keyword is immediately executing the constructor function inside obj? — Rez88
– Rez88, Commented Nov 16, 2020 at 22:38

code-gorilla · Accepted Answer · 2020-11-16 22:38:40Z

1

You declare a property on your object named dog, the expression new Dog(...) is evaluated immediately. This is why you only see one log as the constructor is only called once.

This is a version that would call the constructor 5 times:

const obj = {
  // Notice the use of a function here. 
  dog: () => new Dog('roofus', 20)
}

for(let i = 0; i < 5; i++){
    // Call the function here.
    console.log(obj.dog())
}

answered Nov 16, 2020 at 22:38

code-gorilla

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