check and uncheck using javascript code example

Example 1: checked unchecked through js

document.getElementById("checkbox_id").checked;
//If checked returns true;
//else returns false;

Example 2: check if checkbox is checked javascript

<script type=text/javascript>
  function validate(){
    if (remember.checked == 1){
      alert("checked") ;
    } else {
      alert("You didn't check it! Let me check it for you.")
    }
  }
</script>

<input id="remember" name="remember" type="checkbox" onclick="validate()" />