What is the opposite of !!this.checked?

I am trying to uncheck a box when another is checked. I have it all working except I need to know the opposite of this.checked as well as the opposite of !!this.checked.

Basically I want to use my present method which works rather than change all my code for a new method, so I just need to know the opposite.

this.checked checks the checkbox, but I want the opposite so when it runs it UNchecks it.

var chk1 = $("#theIDofmyCheckbox1");
var chk2 = $("#theIDofmyCheckbox2");

chk1.change(function(){
  chk2.prop( "checked", false );
  // need to set local storage so it saves
  chrome.storage.local.set({'theIDofmyCheckbox2': !!this.checked});
});