Javascript 'new' keyword within function

Question

var steve = function() {
  this.test = new function() {
    console.log('new thing');
  }

  this.test_not_repeating = function() {
    console.log('not repeating');
  }
}; 

steve.prototype.test = function() {
  console.log('test');
};

for (var i = 0; i < 100; i++) {
  var y = new steve();
}

Why does the new keyword force the function to be evaluated X times, where not using the new keyword doesn't? My rudimentary understanding of javascript is, if you do not put the function on the prototype it will be evaluated X times regardless of the new keyword or not.

Is that all of the code? I can see immediately that you're missing a closing brace in the constructor function. Probably off-topic from your question, but still, it stands out. — Antiga
– Antiga, Commented Dec 9, 2015 at 16:40
the closing bracket is in the line steve.prototype.test = function () { console.log('test'); }; The styling is just bad — Safari
– Safari, Commented Dec 9, 2015 at 16:40
@Safari no? That's closing that anonymous function. It's still not closing the constructor (steve) function. — Antiga
– Antiga, Commented Dec 9, 2015 at 16:41

Andy E · Accepted Answer · 2015-12-09 16:50:55Z

3

This function is actually called as a constructor by the new operator, assigning the resulting object to this.test:

this.test = new function() {
    console.log('new thing');
}

This function only gets assigned to this.test_not_repeating, it's never called:

this.test_not_repeating = function() {
    console.log('new thing');
}

Remember that functions do not need parenthesis when called with new:

new Constructor;

// Identical to
new Constructor();

new function () {};

// Identical to
new function () {}();

edited Dec 9, 2015 at 16:50

answered Dec 9, 2015 at 16:45

Andy E

346k86 gold badges482 silver badges452 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

gh9 Over a year ago

which is better ? assigning a function without new or putting the function on the prototype?

Andy E Over a year ago

@gh9: if the function doesn't need the benefit of the enclosing scope, it's usually best to put it on the prototype.

Collectives™ on Stack Overflow

Javascript 'new' keyword within function

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related