"confirm" in function. How can "confirm" starts if it's located only inside "if"?

Here is the code. Could you please explain how the "confirm" works here. It just says, that: "if there is the "confirm" but the is no command "confirm" in fact.

function ask(question, yes, no) {
    if (confirm(question)) {
        yes()
    }
    else {
        no();
    }
  }
function showOk() {
  alert( "You are agree." );
}

function showCancel() {
  alert( "You cancelled." );
}
ask("Are you agree?", showOk, showCancel);