Text Input: Limit input to numbers (0-9) and the minus sign (-). Not working as expected

I am trying to limit keyboard input in my text field to numbers [0-9] and the minus sign - only (no copy/paste, etc.) and the delete key obviously.

The code works for limiting to numbers and the delete key but it doesn’t work for the minus sign - part.

The user should only be able to enter a minus sign - in front of their number, if they try to enter 1 then - it should not input the - but right now the - part doesn’t work at all.

Fiddle: http://jsfiddle.net/7XLqQ/1/

I think this piece of code is the issue but it looks fine. It checks that the text input is blank and if so it input the minus sign -.

// Only enter the minus sign (-) if the user enters it first
if (unicode == 45 && input.value == "") {
    return true;
}

My full code:

<input type="text" maxlength="10" id="myInput">

<script>
var input = document.getElementById("myInput");

input.onkeypress = function(e) {
   var unicode = e.keyCode;

    if (unicode == 49 || unicode == 50 || unicode == 51 || unicode == 52 || unicode == 53 || unicode == 54 || unicode == 55 || unicode == 56 || unicode == 57 || unicode == 48) {
        return true;
    } else {
        return false;   
    }

    // Only enter the minus sign (-) if the user enters it first
    if (unicode == 45 && input.value == "") {
        return true;
    }
};
</script>

Answer

I’d suggest:

var input = document.getElementById("myInput");

input.onkeypress = function(e) {
    switch (e.keyCode){
        case 45:
            return this.value.length == 0 ? true : false;
            break;
        case 48:
        case 49:
        case 50:
        case 51:
        case 52:
        case 53:
        case 54:
        case 55:
        case 56:
        case 57:
            return true;
            break;
        default:
            return false;
            break;
    }
};

JS Fiddle demo.

The reason your original code failed is simply that you’d already returned from the function before the if condition could be assessed. In this version if the - key is pressed a ternary returns true if there is no current value (so the - will be the first character), or false if there is already a value (and therefore the - will not be the first character).