JavaScript: using constructor without operator 'new'

Question

Please help me to understand why the following code works:

<script>
    var re = RegExp('\\ba\\b') ;
    alert(re.test('a')) ;
    alert(re.test('ab')) ;
</script>

In the first line there is no new operator.

As far as I know, a contructor in JavaScript is a function that initialize objects created by the operator new and they are not meant to return anything.

Answer 1

In general, if something is documented as being a constructor, use new with it. But in this case, RegExp has a defined "factory" behavior for the situation where you've called it as a function instead. See Section 15.10.3 of the ECMAScript (JavaScript) specification (that links to the outgoing spec; the section number is the same in the new spec, which you can download from the ECMA front page [on the right-hand side]; I don't want to directly link to a ~4MB PDF file):

15.10.3 The RegExp Constructor Called as a Function
15.10.3.1 RegExp(pattern, flags)
If pattern is an object R whose [[Class]] property is "RegExp" and flags is undefined, then return R unchanged. Otherwise call the RegExp constructor (15.10.4.1), passing it the pattern and flags arguments and return the object constructed by that constructor.

You can actually define your own JavaScript constructor functions to allow omitting the new keyword (by detecting they've been called as a function instead, and turning around and calling themselves correctly), but I wouldn't suggest it as it leads to misleading code.

Answer 2

+1 TJ Crowder has it. The ECMAScript standard goes out of its way to define behaviours for the built-in constructor functions when called as plain functions. Often it simply calls itself back as a constructor, but there are some more complicated cases.

constructors in javascript [...] are not meant to return anything

In general, a constructor can ignore this and just return an independent object:

function Thing() {
    return {'foo': 1};
}

in this case you can equally use the function as a constructor (with new) or a plain function.

If the constructor doesn't return anything, as is the usual pattern for constructors, the new operator itself ensures that it returns the new object created and passed as this. In this case you must use new.

It's best not to rely on a constructor working as a bare function, and the alternative behaviours of the built-in constructors are rarely of any use, so generally you should stick with new.