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Recently, I have got into a habit of calling things like RegExp, String, Number, Object, TypeError, etc without "new".

e.g:

throw (TypeError("Error"));
var regex = RegExp('^word$');

I know that this is bad for cases when "this" context is needed, since without "new", "this" wreaks havoc on your global scope unless you wrap your code in 'use strict', in which case it raises an error that you are trying to mutate 'undefined'. (I am not sure that this works in very very old browsers).

e.g:

var constructor = function() {
    // 'use strict'; /* uncomment this line to avoid the behavior and be warned */
    this.state = 'working as intended';
};

var foo = constructor();
console.log(foo.state); // undefined
console.log(window.state); // we just polluted our global scope.

whereas 

var constructor = function() {
    this.state = 'working as intended';
};

var foo = new constructor;
console.log(foo.state); // "working as intended"
console.log(window.state); // we are clean.

But in cases like the ones above, is it okay to do this, or are there problems I am setting myself up for if I get into a habit of doing this?

Thanks ahead of time.

Dmytro
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2 Answers2

8

Be aware the result can be different.

For example, the Number constructor creates Number objects, but when called as a function it only does type coercion to primitive Number.

new Number(123); // Number { 123 }
Number(123); // 123

But yes, there are lots of the cases in which it doesn't matter whether you use new or not. They exist because of backwards compatibility, but with recently introduced constructors like Set or Map do require new.

In general, I would recommend using new when you want to create a new object. Then,

  • If you only want to do type coercion, you must call Boolean, Number or String without new.
  • For the same reason, if you want coerce to Object type, I wouldn't use new, but it doesn't matter.
  • If you want to create a primitive which doesn't have literal syntax, you must call Symbol without new.
  • If you want to create an object wrapper of a primitive value, you must call Boolean, Number, String or Symbol with new.
  • If you want to instantiate an old constructor like Array, Object, RegExp, Error, etc., I would use new, but it doesn't matter.
  • If you want to instantiate a recently introduced constructor like Set, Map, WeakSet, WeakMap, typed arrays, etc., you must call it with new.

For the old constructors that it doesn't matter, it's like they call themselves with new if you omit it. For example, for RegExp,

When RegExp is called as a function rather than as a constructor, it creates and initializes a new RegExp object. Thus the function call RegExp(…) is equivalent to the object creation expression new RegExp(…) with the same arguments.

Oriol
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  • So you are saying for things like RegExp and TypeError, and String(for concatenation context), it is perfectly fine? – Dmytro Aug 22 '16 at 20:02
  • @Dmitry It's fine for [The RegExp Constructor](http://www.ecma-international.org/ecma-262/6.0/#sec-regexp-constructor) and [The Error Constructor](http://www.ecma-international.org/ecma-262/6.0/#sec-error-constructor). Not for String, Boolean, Number. – Oriol Aug 22 '16 at 20:04
  • so doing things like `console.log(String(1392))` instead of `console.log(1392.toString())` or `console.log('' + 1392)` or just `console.log(1392)` is not okay? – Dmytro Aug 22 '16 at 20:08
  • @Dmitry Ah, no, I meant `new String(1392)` and `String(1392)` do different things. Usually you want `String(1392)`. – Oriol Aug 22 '16 at 20:12
  • It's okay if it does what you want. If what you want is to create an object, you'll have to either use `new` or check the documentation to make sure the call without `new` creates an object. – Aaron Aug 22 '16 at 20:12
  • I am now kinda curious how String, RegExp, TypeError create objects even though they are not called with new. Do they call new within themselves? Nevermind, `var a = RegExp(); console.log(a.prototype);` is undefined, so it wasn't created with new. Double nevermind, even with new it's undefined. O.o – Dmytro Aug 22 '16 at 20:15
  • @Dmitry Only constructors have a `prototype` property. – Oriol Aug 22 '16 at 20:19
3

As documented in another answer, some built in constructors are written to be able to be called as functions. So as long as you aren't polluting your global namespace, I believe you should be fine with the example you listed.

That being said, in most cases, I wouldn't get used to this habit; the new operator adds to code clarity, which is more important than brevity. The ability to use native constructors without the new operator is also inconsistently applied across constructors.

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Kyle Baker
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