Is javascript new keyword optional for javascript API feature?

Question

I having trouble to understand why the new keyword is facultative for javascript API object/interface feature.

d = new String(); // javascript native object
d2 = String();
console.log(d);
console.log(d2);

results in console (that seems pretty normal):

 String {}
 (an empty string)

but:

b = new Blob(); // API object
b2 = Blob();
console.log(b);
console.log(b2);

results:

Blob { size=0, constructor=function(), type="", more...}
Blob { size=0, constructor=function(), type="", more...}

and not:

Blob { size=0, constructor=function(), type="", more...}
Blob() is undefined or (an empty blob)

It all work just fine but I'm curious...

I guess those just internalize the new instance within them if you omit it. make your own factory function and try the same. — vsync
– vsync, Commented Oct 6, 2015 at 20:14

Community · Accepted Answer · 2017-05-23 10:26:59Z

2

It's a wart in JS. There are functions that construct objects internally and return them (don't need new), and "constructor functions" using new that do a bunch of magic.

The usual recommendation is to never write your own types to require new, because new requiring functions do bad but subtle things if you omit it (specifically, they bind this to the global object instead of a new object, so the instance variables are really the global variables, and assigning to instance members clobbers globals). If you must use them, JavaScript: The Good Parts recommends using initial capitals in the name to mean "requires new" and initial lowercase for everything else.

edited May 23, 2017 at 10:26

CommunityBot

11 silver badge

answered Oct 6, 2015 at 20:28

ShadowRanger

158k12 gold badges221 silver badges315 bronze badges

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

5 Comments

TGrif Over a year ago

They both add object to the global Window (maybe because my browser isn't so recent)

ShadowRanger Over a year ago

It's not about whether the result of the assignment is placed on the global window (usual case for variables not declared with var). It's whether assignments internal to the constructor use a this of the new object being constructed (using new) or use a this that's the same this the caller had (usually the global window, though not always). If I define var Foo = function(x) { this.ab = x; }, then do var f1 = new Foo("123"); then f1 is a new object with attribute ab set to "123". If I forget the new, then f1 is undefined and there is a global ab set to "123".

ShadowRanger Over a year ago

By constrast, a constructor-like function not designed to use new would do all the work internally and return the newly constructed object; it doesn't rely on new having been used to give it a special this. Since the two forms are mostly incompatible, you have to choose, and the non-new form is more flexible, and has less surprising side-effects when misused.

ShadowRanger Over a year ago

There are things you can do to try and handle both cases, but they can't handle all the corner cases, and they're generally just confusing, so the usual approach is to stick to only one method of writing constructors (usually w/o new), except in those annoying cases where the browser builtins require new usage.

TGrif Over a year ago

Thank you a lot for your great explaination.

Collectives™ on Stack Overflow

Is javascript new keyword optional for javascript API feature?

1 Answer 1

5 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Linked

Related