JavaScript - Using “checked/unchecked checkbox” as “IF ELSE condition statement”

问题

Now I want to use "check/unchecked checkbox" as "IF condition statement".

Example: I want to make a chord analyzer based on the note(s) you hit. When you check a note (or some notes) on checkbox, then press the button under those checkboxes, the paragraph element should tell you about what chord you hit. But there's error on my writing at if ("error") statement. How to fix it?

Here's my code:

<!doctype html>
<html>
   <head>
      <script>
       function fungsi() {

         var c = document.getElementById("C").checked;
         var e = document.getElementById("E").checked;
         var f = document.getElementById("F").checked;
         var g = document.getElementById("G").checked;

         if (c.checked = true &&
           e.checked = true &&
           g.checked = true &&
           f.checked = false) {
           document.getElementById("chord").innerHTML = "C Chord";
         } else if (c.checked = true &&
           f.checked = true &&
           g.checked = false &&
           e.checked = false) {
           document.getElementById("chord").innerHTML = "F Chord";
         } else {
           alert("Under Development!");
         }
       }
      </script>
   </head>
   <body>
      <form>
         Note(s) That You Hit:<br>
         <input type=checkbox id="C">C<br>
         <input type=checkbox id="E">E<br>
         <input type=checkbox id="F">F<br>
         <input type=checkbox id="G">G
      </form>
      <button type=button onclick="fungsi()">The Chord Is</button>
      <p id="chord"></p>
   </body>
</html>

回答1:

if (c.checked=true) and similar should be simply if (c.checked).

if (f.checked=false) and similar should be simply if (!f.checked).

If you want, you can do if (c.checked==true) or if (c.checked===true), and similarly if (f.checked==false) or if (f.checked===false), but it's pointless the vast majority of the time. You already have a boolean value. Just if (c.checked) and if (!f.checked) is sufficient.

E.g.:

if (c.checked &&
    e.checked &&
    g.checked &&
    !f.checked) {
    document.getElementById("chord").innerHTML = "C Chord";
}

Separately, as crclayton points out, you either want:

var c=document.getElementById("C").checked;
var d=document.getElementById("E").checked;
// ...and so on..., and then:
if (c && e && g && !f)

var c=document.getElementById("C"); // <== No .checked
var d=document.getElementById("E"); // <==
// ...and so on..., and then:
if (c.checked &&
    e.checked &&
    g.checked &&
    !f.checked) {

来源：https://stackoverflow.com/questions/32589660/javascript-using-checked-unchecked-checkbox-as-if-else-condition-statement

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javascript

checkbox