I really think you'll be able to do what you are doing with Field Rules so much easier than Javascript - the field rules in the Modern Designer are so much more powerful than in the Classic Designer.

That said - here is some Javascript with a lot of detailed comments regarding how to work with LFForm.getFieldValues and values from a checkbox field.  This was tested in Version 11.0.2311.50556. 

//Let's assume we have a checkbox with variable
//of showWeeknd that has checkboxes for Saturday
//and Sunday.
LFForm.onFieldChange(function() {

  //Variable declaration.
  let isChecked = ''; 
  
  //This should return an object with details from the checkbox.
  //the console will show the following:
    //  no checks:         an object with a child object named value (with 0 elements)
    //  Saturday checked:  an object with a child object named value (with 1 elements)
    //  Sunday checked:    an object with a child object named value (with 1 elements)
    //  Both checked:      an object with a child object named value (with 2 elements)
  isChecked = LFForm.getFieldValues({variableName: "showWeeknd"});
  console.log('1: ');
  console.log(isChecked);
  
  //This should return an array of the values from the checkbox.
  //the console will show the following:
    //  no checks:         {empty string}
    //  Saturday checked:  Saturday
    //  Sunday checked:    Sunday
    //  Both checked:      Saturday, Sunday
  isChecked = LFForm.getFieldValues({variableName: "showWeeknd"}).value;
  console.log('2: ' + isChecked);
  
  //This should return the first value from the checkbox.
  //the console will show the following:
    //  no checks:         {undefined}
    //  Saturday checked:  Saturday
    //  Sunday checked:    Sunday
    //  Both checked:      Saturday
  isChecked = LFForm.getFieldValues({variableName: "showWeeknd"}).value[0];
  console.log('3: ' + isChecked);
  
  //This is the same as the second one, but will loop through 
  //each item in the array.
  //the console will show the following:
    //  no checks:         {does not run}
    //  Saturday checked:  Saturday
    //  Sunday checked:    Sunday
    //  Both checked:      Saturday
    //                     Sunday
  isChecked = LFForm.getFieldValues({variableName: "showWeeknd"}).value;
  isChecked.forEach(function(element) {
    console.log('4: ' + element);
  });
  
  //Put a break into the log.
  console.log('-----------------');
  
//End of LFForm.onFieldChange(function() {
}, {variableName: "showWeeknd"});