function F() { if (!(this instanceof F)) { return new F() }; … }

Question

What is the usage of the construct: function F() { if (!(this instanceof F)) { return new F() }; ... }?

I found this in a pty.js for Node.

Answer 1

It means that if the function was called without the new operator, it will automagically return a new instance.

For example, if you didn't have this safeguard, and did this...

var t = Terminal();

...then the this while executing Terminal() would point to window (or your global object, fancy non-browser guy/gal), definitely not what you want.

By determining that this is in fact an instance of Terminal, then we can proceed. Otherwise, the safeguard returns a new object.

Then we can simply use both forms...

var t = Terminal(); // Will be same as `new Terminal()`