如何成功突破javascript中的功能？

Question

So I have a function that checks if the last character in a string is an arithmetic operator(+,-,*,/), and if it is it should immediately return from the function. If not, the subtract sign should be appended to the string. However, only the second case seems to be occurring and I can't figure out why. If anyone could provide some insight I would be very appreciative.

    $("#subtract").click(function () {
        var original=$("#display").text();
        var sliced=original.slice(0,original.length - 1);
        var signs=["+","-","*","/"];
        var charpos=sliced.charAt(sliced.length -1);

        if ((charpos === signs[0]) || (charpos === signs[1]) ||  (charpos  === signs[2]) || (charpos === signs[3])) {
            return;
        }
        else {
            var newdisplay=sliced + "-";
            $("#display").text(newdisplay);
        }

     });

Answer 1

var original=$("#display").text();
var sliced=original.slice(0,original.length - 1);

the second line gets rid of the last character in your string. so you're not even able to check what that was. just get rid of it and change the first line to

var sliced=$("#display").text();

and everything should work fine

Answer 2

Instead of

if ((charpos === signs[0]) || (charpos === signs[1]) ||  (charpos  === signs[2]) || (charpos === signs[3])) {
        return;
    }

try

if ($.inArray(charpos, signs) > -1) {
    return;
}

First of all, never enumerate items from an array in such a fashion - it's easy to miss items that way. Secondly, exact instance comparison (===) won't fire, unless you compared exact same strings.

Besides, consider revising how you extract the last character.

Answer 3

You're slicing the input which is removing the last character, then using charAt and getting the second to last character.

var original=$("#display").text(); // "foobar"
var sliced=original.slice(0,original.length - 1); // "fooba"
var charpos=sliced.charAt(sliced.length -1); // "a" -- we lost the "r"

You only need to call charAt and work with the last character (or, if you prefer slice , it's able to accept a negative number and backtrack the string).

Secondly, since signs is an array, you can use indexOf over checking each element. eg

var original = $('#display').text();
var signs = ["+","-","*","/"];
var lastChar = original.slice(-1); // OR original.charAt(original.length - 1);

// Check for the lastChar in the signs array
if (signs.indexOf(lastChar) != -1){ // -1 == not found
  return; // short exit
}