Question

I’m working with a JavaScript routine I didn’t write. It is called from a text box’s onkeydown attribute to prevent unwanted keystrokes.

The first argument is apparently not used. The second argument is a list of characters that should be allowed.

function RestrictChars(evt, chars) {
    var key;
    var keychar;

    if (window.event)
        key = window.event.keyCode;
    else if (e)
        key = e.which;
    else
        return true;

    keychar = String.fromCharCode(key);

    if ((key == null) || (key == 0) || (key == 8) ||
        (key == 9) || (key == 13) || (key == 27))
        // Control key
        return true;
    else if (((chars).indexOf(keychar) > -1))
        return true;
    else
        return false;
}

This seems to work for alpha-numeric characters. However, characters such as . and / cause this function to return false, even when these characters are included in the chars parameter. For example, if the . key is pressed, key is set to 190, and keychar gets set to the “3/4” character.

Can anyone see how this was meant to work and/or why it doesn’t? I don’t know enough about JavaScript to see what it’s trying to do.

Answer 1

Two things are wrong with that: first, if you’re analysing what character has been typed, you need to use the keypress event instead of keydown because that is the only event that tells you anything reliable about the actual character typed. For (a lot) more detail about about this and JavaScript key events in general, see http://unixpapa.com/js/key.html. Second, there are references to a variable called e which doesn’t (but should) correspond with the evt parameter.

Here’s a rewrite, assuming you have a variable called textBox that refers to the text input element.

jsFiddle: http://jsfiddle.net/9DZwL/

Code:

function isKeypressCharValid(e, chars) {
    e = e || window.event;

    // Allow delete, tab, enter and escape keys through
    if (/^(8|9|13|27)$/.test("" + e.keyCode)) {
        return true;
    }

    var charCode = (typeof e.which == "number") ? e.which : e.keyCode;
    var charTyped = String.fromCharCode(charCode);
    return chars.indexOf(charTyped) > -1;
}

textBox.onkeypress = function(evt) {
    if (!isKeypressCharValid(evt, "abc123")) {
        return false;
    }
};