Validate input value before it is shown to user

Question

I have an html <input> and some pattern (eg -?\\d*\\.?\\d* float-signed value).
I should prevent typing the not matched value.
I did it in next way

jQuery.fn.numeric = function (pattern) 
{
   var jqElement = $(this), prevValue;
    jqElement.keydown(function()
    {
                          prevValue = jqElement.val();
    })
    jqElement.keyup(function(e)
    {
       if (!pattern.test(jqElement.val()))
       {
          jqElement.val(prevValue);
          e.preventDefault();
       }
       prevValue = ""
    }) 
};  

JSFiddle DEMO

But in this case, value is shown to user and then corrected to right value.
Is it way to vaidate value before it is shown to user?

I can use pattern attribute from html5

Answer 1

$("#validateMe").on('keydown', function() {
    var charBeingTyped = String.fromCharCode(e.charCode || e.which); // get character being typed
    var cursorPosition = $(this)[0].selectionStart;        // get cursor position
    // insert char being typed in our copy of the value of the input at the position of the cursor.
    var inValue = $(this).value().substring(0, cursorPosition) + charBeingTyped + $(this).value().substring(cursorPosition, $(this).value().length);
    if(inValue.match(/-?\d*\.?\d*/)) return true;
    else return false;
});

Answer 2

How about this POJS, I'm using a cross-browser addEvent function instead of jquery and not using any regexs, but I believe it achieves what you are looking for. Pressing + or - changes the sign of the value.

HTML

<input id="test" type="text" />

Javascript

/*jslint maxerr: 50, indent: 4, browser: true */


(function () {
    "use strict";

    function addEvent(elem, event, fn) {
        if (typeof elem === "string") {
            elem = document.getElementById(elem);
        }

        function listenHandler(e) {
            var ret = fn.apply(null, arguments);

            if (ret === false) {
                e.stopPropagation();
                e.preventDefault();
            }

            return ret;
        }

        function attachHandler() {
            window.event.target = window.event.srcElement;

            var ret = fn.call(elem, window.event);

            if (ret === false) {
                window.event.returnValue = false;
                window.event.cancelBubble = true;
            }

            return ret;
        }

        if (elem.addEventListener) {
            elem.addEventListener(event, listenHandler, false);
        } else {
            elem.attachEvent("on" + event, attachHandler);
        }
    }

    function verify(e) {
        var target = e.target, // shouldn't be needed: || e.srcElement;
            value = target.value,
            char = String.fromCharCode(e.keyCode || e.charCode);

        if (value.charAt(0) === "-") {
            if (char === "+") {
                e.target.value = value.slice(1);
            }
        } else if (char === "-") {
            e.target.value = char + value;
            return false;
        }

        value += char;
        return parseFloat(value) === +value;
    }

    addEvent("test", "keypress", verify);
}());

On jsfiddle

I think I used the correct values keyCode || charCode keyCode || charCode but you may want to search and check. A summary of the correct ones are available here

Answer 3

You could use this code to find out what character is pressed. Validate that character and, if it validates, append it to the input field.

Answer 4

Try this code:

 jQuery.fn.numeric = function (pattern) { $(this).keypress(function(e) { var sChar = String.fromCharCode(!e.charCode ? e.which : event.charCode); e.preventDefault(); var sPrev = $(this).val(); if(!pattern.test(sChar)){ return false; } else { sPrev = sPrev + sChar; } $(this).val(sPrev); }); }; $("#validateId").numeric(/^-?\\d*\\.?\\d*$/); 

jsfiddle.net/aBNtH/

UPDATE: My example validates each charachter while typing. If you prefer to check the entire value of the input field instead, I would suggest to validate the value on an other Event, like Input blur().