From a short review;

• You should declare your variables with const or let

• There is a ton of copy-pasted code in there

• 'commission-field' is not a great name for your element, I would just call it commission

• Avoid writing multiple calls to document.getElementById for the same id

• Its hard to control what users enter, and sometimes parseFloat returns a NaN, I would always deal with that pro-actively

• I would create a function that formats an element like

  function cleanAmountElement(id){
    const elementValue = document.getElementById(id);
    const cleanerValue = parseFloat(elementValue*1).toFixed(2);    
    document.getElementById(id).value = IsNan(cleanerValue)?0:cleanerValue;
    return cleanerValue;
  } 
  

• Note that this is not super super clean, that function both updates the UI and returns a value :/

• Then you can do something funky with functional programming where you pass a list of ids and process them

  const sum = ['salary', 'commission', 'rental', 'other_income'].map(cleanAmountElement)
                                                                .reduce(getSum,0);
  
  function getSum(a,b){
    return a+b;
  }
  

From a short review;

• You should declare your variables with const or let

• There is a ton of copy-pasted code in there

• 'commission-field' is not a great name for your element, I would just call it commission

• Avoid writing multiple calls to document.getElementById for the same id

• Its hard to control what users enter, and sometimes parseFloat returns a NaN, I would always deal with that pro-actively

• I would create a function that formats an element like

  function cleanAmountElement(id){
    const elementValue = document.getElementById(id);
    const cleanerValue = parseFloat(elementValue*1).toFixed(2);
    const cleanestValue = IsNaN(cleanerValue)?0:cleanerValue;    
    document.getElementById(id).value = cleanestValue;
    return cleanestValue;
  } 
  

• Note that this is not super super clean, that function both updates the UI and returns a value :/

• Then you can do something funky with functional programming where you pass a list of ids and process them

  const sum = ['salary', 'commission', 'rental', 'other_income'].map(cleanAmountElement)
                                                                .reduce(getSum,0);
  
  function getSum(a,b){
    return a+b;
  }