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

by

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()`

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