Find the sum of a Javascript array and divide by its length -

i'm embarrassed ask this.

i'm beginner programmer, , javascript confusing me. managed put of instructor, there simple things can't right.

i tried search stack overflow thread answer question, of them i've seen contain code haven't learned yet, they're gibberish me.

what i'm trying add values of array , divide sum array's length, ergo, find average. description of assignment find average of number of students' grades.

my 2 problems

1. i can't figure out how sum of numeric values in array and,
2. for reason, array.length returns 1 more actual length of array, if add -1. (ex. if enter 6 values, array.length return 7.)

i know problem can't figure out need enter. assignment due tomorrow, anyone's time , effort appreciated.

here script:

<script type="text/javascript">     var allgrades = new array();     var g = 0;     var l = 0;     var s = 0;     var t = 0;     {         allgrades[g] = window.prompt("please enter 1 grade each window. after enter grade, enter 'x' see average of grades entered.", "")         g++;     }      while (allgrades[g - 1] != "x")      (l = 0; l < allgrades.length - 1; l++) {         s += allgrades[l] // think problem     }     t == s / g - 1;     g == allgrades.length - 1; //     window.alert(g)      switch (t) {         case (t >= 90):             window.alert("your average grade " + (t) + ". " + "this a.")             break;         case (t >= 80 && t < 90):             window.alert("your average grade " + (t) + ". " + "this b.")             break;         case (t >= 70 && t < 80):             window.alert("your average grade " + (t) + ". " + "this c.")             break;         case (t >= 60 && t < 70):             window.alert("your average grade " + (t) + ". " + "this d.")             break;         case (t <= 60):             window.alert("your average grade " + (t) + ". " + "this failing grade.")             break;     } </script> 

i'm sorry if i'm asking seems dumb. i've been taking web programming 2 months, use help!

kyle

== comparison operator. need use assignment operator (=) here:

t==s/g-1;  

and lines near it.

also, own sake, not use single-letter variable names unless have reason doing so.

here's cleaner way of writing script:

var grades = [];  {     var input = window.prompt("please enter 1 grade each window. after enter grade, enter 'x' see average of grades entered.", "");      grades.push(parsefloat(input)); } while (input != 'x');  var sum = 0;  (int = 0; < grades.length; i++) {     sum += grades[l]; }  var average = (sum / grades.length) * 100; var grade;  if (average >= 90) {     grade = 'a'; } else if (average >= 80) {     grade = 'b'; } else if (average >= 70) {     grade = 'c'; } else if (average >= 60) {     grade = 'd'; } else {     grade = 'failing grade'; }  alert('your average grade ' + average + '. ' + 'this ' + grade);