Call function directly after constructor "new"

internals, javascript

Solution

In many languages, to invoke some function directly on new instance I need to wrap it into braces Is it allowed to use function call immediately after instantiating new object in this way?

I think the problem here is potential ambiguity. While `(new Date()).toString()` is clear, `new Date().toString()` could (theoretically) mean either `(new Date()).toString()` or `new (Date().toString)()`.

But here comes operator precedence to the rescue.

the `new (with argument list)` operator has a higher precedence than a function call. Therefore the order of execution in `new Date().toString()` is as clear (at least to the engine) as the order in wich these math operations have to be executed `2 + 3 * 4`

But beware, in JS you can also write `new Date;` without the brackets, and this operator `new (without argument list)` has a lower precedence than a function call. So `new Date.toString()` is interpreted as `new (Date.toString)()` and you get an Error that `toString()` is no constructor.

It looks like new keyword is optional sometimes. How is this interpreted by JS engine?

Not at all. This ain't a JS feature this is a quirk in the implementation of these functions/classes/constructors

A bit like these implementations:

function Foo(){
    if(!(this instanceof Foo)) 
        return new Foo();

    this.foo = 42;
}

//or something like
function Bar(){
    return Object.create(Bar.prototype, {
        bar: { value: 42 }
    });
}

console.log(new Foo(), Foo());
console.log(new Bar(), Bar());

And again beware. Due to added security/failsafety you can't implement something like that with the new `class` keyword. It adds (hidden) code to ensure that these constructors are always called with the `new` keyword, otherwise the constructor throws an error.

Edit: For example the Array constructor works like described above, but as it seems (as pointed out in Freeman Lambdas answer) the Date constructor has (defined by the spec) two different return values, depending on wether you call it as a function or as a constructor.

Problem

Original source