How does the 'selector' get initialized

function FormHandler(selector) {
if (!selector){
throw new Error(‘No selector provided’); //path taken and selector undefined
}

See book page 214 - 217 – to this point all is working, but I do not understand how the selector
is initialized? Not sure if there is missing code or what? I look for the initialization of the selector
but I cannot find it or not sure how it gets its values?

Thanks, M