Invoking javascript from your page:

  <a ... onclick="yourCodeHere();"> ... </a>

For forms:
  <form onsubmit="return allValidates();">...</form>

AllValidates() should call:
   validateFirstName()
   validateShoeSize();
   etc

AND the input for the firstName will have
  <input ... onblur="validateFirstName();">...</input>
AND the input for the shoeSize will have
  <input ... onblur="validateShoeSize();">...</input>

This way we won't repeat code, and yet we call it both
  onblur of individual items, and onsubmit of the entire form.



Also (good structure): 
  the individual 'validate' functions will do two things:
  - determine correctness and return a boolean-ish value
  - AND, if it didn't validate, it will put an error message up to the user.
    [Presumably by adding a new node to the DOM, as in prev. lect.]

Note that 'create/display an error message for shoeSize' and
          'create/display an error message for firstName' etc
shouldn't repeate code; that's why the hw also asks for a common helper-function
   'displayErrorMessage' -- it'll be passed a string, and might be called by
   validateShoeSize and by validateFirstName etc.

                    validateShoeSize  - displayErrorMessage
                  /
      validateAll - validateCarModel  - displayErrorMessage
                  \
                    validateFirstName - displayErrorMessage


  For displayErrorMessage -- WHERE in the DOM should it add the message?
  You'd like the error message to be located near the 'input' node of the DOM.
  My sol'n: pass displayErrorMessage *two* things -- both the string to display,
  *and* the DOM-node that failed-to-validate.
  Then, displayErrorMessage can locate that node, find its parent,
  and insert a new-textnode into the DOM right next to the offending node.

  E.g.    
    function validateShoeSize(theShoeSizeInutNode) { 
         if (theShoeSizeInutNode.value >= 200)  {
             displayErrorMessage("your feet are HUGE", theShoeSizeInutNode);
             return false;
             }
         else {
             return true;
             }
         }
  Then, when calling it, pass in the node:
     <input ... onblur="validateShoeSize(this);">...</input>


  Q: What's wrong with:
        function validateAll() {
            return validateShoeSize(...)
                && validateFirstName(...)
                && validateCarModel(...);
           }
  A: because of short-circuiting, if there are multiple errors you might not
     cause all of them to be displayed.
  Soln: 
        function validateAll() {
            var isValid = true;
            isValid = isValid && validateShoeSize(...);
            isValid = isValid && validateFirstName(...);
            return isValid;
            }

            (gee, I wish I could write `isValid &&= validateShoeSize()`, but `&&=` isn't 
             an operator even though `+=` is.)