if checkbox is checked, do this

When I check a checkbox, I want it to turn <p> #0099ff.

When I uncheck the checkbox, I want it to undo that.

Code I have so far:

$('#checkbox').click(function(){
	if ($('#checkbox').attr('checked')) {
        /* NOT SURE WHAT TO DO HERE */
	}
}) 

I would use .change() and this.checked:

$('#checkbox').change(function(){
    var c = this.checked ? '#f00' : '#09f';
    $('p').css('color', c);
});

--

On using this.checked
Andy E has done a great write-up on how we tend to overuse jQuery: Utilizing the awesome power of jQuery to access properties of an element. The article specifically treats the use of .attr("id") but in the case that #checkbox is an <input type="checkbox" /> element the issue is the same for $(...).attr('checked') (or even $(...).is(':checked')) vs. this.checked.

Try this.

$('#checkbox').click(function(){
    if (this.checked) {
        $('p').css('color', '#0099ff')
    }
}) 

Sometimes we overkill jquery. Many things can be achieved using jquery with plain javascript.

It may happen that "this.checked" is always "on". Therefore, I recommend:

$('#checkbox').change(function() {
  if ($(this).is(':checked')) {
    console.log('Checked');
  } else {
    console.log('Unchecked');
  }
});

it's better if you define a class with a different colour, then you switch the class

$('#checkbox').click(function(){
    var chk = $(this);
    $('p').toggleClass('selected', chk.attr('checked'));
}) 

in this way your code it's cleaner because you don't have to specify all css properties (let's say you want to add a border, a text style or other...) but you just switch a class

I found out a crazy solution for dealing with this issue of checkbox not checked or checked here is my algorithm... create a global variable lets say var check_holder

check_holder has 3 states

1. undefined state
2. 0 state
3. 1 state

If the checkbox is clicked,

$(document).on("click","#check",function(){
    if(typeof(check_holder)=="undefined"){
          //this means that it is the first time and the check is going to be checked
          //do something
          check_holder=1; //indicates that the is checked,it is in checked state
    }
    else if(check_holder==1){
          //do something when the check is going to be unchecked
          check_holder=0; //it means that it is not checked,it is in unchecked state
    }
     else if(check_holder==0){
            //do something when the check is going to be checked
            check_holder=1;//indicates that it is in a checked state
     }
});

The code above can be used in many situation to find out if a checkbox has been checked or not checked. The concept behind it is to save the checkbox states in a variable, ie when it is on,off. i Hope the logic can be used to solve your problem.

Check this code:

<!-- script to check whether checkbox checked or not using prop function -->
<script>
$('#change_password').click(function(){
    if($(this).prop("checked") == true){ //can also use $(this).prop("checked") which will return a boolean.
        alert("checked");
    }
    else if($(this).prop("checked") == false){
        alert("Checkbox is unchecked.");
    }
});
</script>

$('#checkbox').change(function(){
   (this.checked)?$('p').css('color','#0099ff'):$('p').css('color','another_color');
});

Probably you can go with this code to take actions as the checkbox is checked or unchecked.

$('#chk').on('click',function(){
    if(this.checked==true){
      alert('yes');
    }else{
      alert('no');
    }
});

I would do :

$('#checkbox').on("change", function (e){ 
    
    if(this.checked){
    
      // Do one thing 
    
    }
    
    else{
    
     // Do some other thing
    
    }

});

See : https://www.w3schools.com/jsref/prop_checkbox_checked.asp

###Optimal implementation

$('#checkbox').on('change', function(){
    $('p').css('color', this.checked ? '#09f' : '');
});

---

###Demo

<script src="https://code.jquery.com/jquery-1.12.2.min.js"></script>
<input id="checkbox" type="checkbox" /> 
<p>
    Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do
    eiusmod tempor incididunt ut labore et dolore magna aliqua.
</p>
<p>
    Ut enim ad minim veniam, quis nostrud exercitation ullamco
    laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure
    dolor in reprehenderit in voluptate velit esse cillum dolore eu
    fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
    proident, sunt in culpa qui officia deserunt mollit anim id est
    laborum.
</p>

$('#checkbox').on('change', function(){
    $('p').css('color', this.checked ? '#09f' : '');
});

Why not use the built in events?

$('#checkbox').click(function(e){
    if(e.target.checked) {
     // code to run if checked
        console.log('Checked');

     } else {
     
     //code to run if unchecked
        console.log('Unchecked');
     }
});