Because this is not set at function call (using Function.prototype.call and such) it's null, which becomes the global scope at non-strict mode:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this#Function_context

So this is essentially another solution that uses global object.

this.children = this.children || [];

how does this allow the function to retain the value of children between calls?

i thought it was just storing it on the object, but the or is required.