How to check whether a Button is clicked by using JavaScript

Try adding an event listener for clicks:

document.getElementById('button').addEventListener("click", function() {
   alert("You clicked me");
}​);​

Using addEventListener is probably a better idea then setting onclick - onclick can easily be overwritten by another piece of code.

You can use a variable to store whether or not the button has been clicked before:

var clicked = false
document.getElementById('button').addEventListener("click", function() {
   clicked = true
}​);​

addEventListener on MDN

You can add a click event handler for this:

document.getElementById('button').onclick = function() {
   alert("button was clicked");
}​;​

This will alert when it's clicked, if you want to track it for later, just set a variable to true in that function instead of alerting, or variable++ if you want to count the number of clicks, whatever your ultimate use is. You can see an example here.