I\'m trying to pause and then play a setInterval loop.
After I have stopped the loop, the \"start\" button in my attempt doesn\'t seem to work :
JSFiddle wraps your code in a function, so start() is not defined in the global scope.

Moral of the story: don't use inline event bindings. Use addEventListener/attachEvent.
Please don't pass strings to setTimeout and setInterval. It's eval in disguise.
Use a function instead, and get cozy with var and white space:
var input = document.getElementById("input"),
add;
function start() {
add = setInterval(function() {
input.value++;
}, 1000);
}
start();