javascript - Instantly detect an input being checked -

say have input type "checkbox"

<input type='checkbox' id='checker'> 

i need able detect instantly whether box checked, without button click or function call. checking , unchecking box lighten or darken text. how check whether box unchecked or not in vanilla javascript? need able this, preferably without infinite for-loop that'll crash page. know method it

var value=document.getelementbyid('checker').checked; 

but need way see whether has been checked or not without reloading page using vanilla javascript. please help!

edit: considering using 2 radio buttons, 1 value true , other value false, still need way of seeing 1 checked.

check working demo: http://jsfiddle.net/rrath8tg/1/:

// checkbox reference element checkbox.onchange = function() {      if(this.checked) {          // when checked      } }; 

or onchange on checkbox element.