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not receive negative numbers

I have the following code: http://jsfiddle.net/ntywf/1987/
$(document).ready(function () {
$('input').keyup(function() {
var $th = $(this);
$th.val($th.val().replace(/[-]/g, function(str) {
//alert('You typed " ' + str + ' ".\n\nPlease use only letters and numbers.');
return '';
}));
});
});
what I want is to remove the "-" sign off when it is inserted. what happens is that the cursor is always the last decimal home. I just want this code not to let the user enter negative numbers. How can I do this? (the problem is to move the cursor within the input, since it is always sent to the last character)

You can use a KeyCode (Link) to verify what key you pressed, and use replace to remove it:
$('input').keyup(function(e) {
var code = e.keyCode || e.which;
if(code == 109 || code == 189) { //Enter keycode
//Do something
var valor = $(this).val();
$(this).val(valor.replace(/[-]/g, ''))
}
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<input type="text"/>

Here what I have tried.
JS
$('input').keyup(function() {
var $th = $(this).val();
$th = $th.replace(/[-]/g, "");
$(this).val($th)
console.log( $(this).val());
});
It will remove - sign from data.

This should solve your problem
What I have done is:
I have used the inbuilt HTML input field method setSelectionRange(), which sets the start and end positions of the current text selection in an element. (From MDN)
MDN Reference : https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/setSelectionRange
JS Code:
$(document).ready(function () {
$('input').keyup(function() {
var $th = $(this);
$th.val( $th.val().replace(/[-]/g, function(str) {
//alert('You typed " ' + str + ' ".\n\nPlease use only letters and numbers.');
return '';
} ) );
$('input')[0].setSelectionRange(0, 0); //this method sets the range to zero text starting from 0 index to 0 index
});
});
JSFiddle: http://jsfiddle.net/dreamweiver/ntywf/1998/

Use type = "numeric" and min="0" This way you can prevent your text-field from accepting alphabets as well. min=0 will always make sure that it will never accept -ve value.
<input type="number" min="0"/>
JSFIDDLE DEMO will be helpful to you.

Can we use a constructed dynamic jQuery selector?

I want to contruct a jquery selector as a string and pass its value to the selector.
$(document).on( 'keyup', function( e ) {
if( e.keyCode == 9 ) {
//console.log( e.target );
console.log(e.target.id);
var preTabIndex = document.getElementById(e.target.id).tabIndex;
var nextTabIndex = preTabIndex + 1;
console.log(preTabIndex);
console.log(nextTabIndex);
//console.log($('[tabindex=3]')[0].id);
var selector = "[tabindex=" + nextTabIndex + "]";
console.log(selector);
console.log($(selector)[0].Id);
//document.getElementById($("[tabindex=3]")[0].id).focus();
document.getElementById($(selector)[0].id).focus();
}
} );
Can this be done? I couldn't find it on my initial googling.
With this i am getting an undefined when i do
console.log($(selector)[0].Id);

Yes, you can. Make sure you use the . or # to denote the target. For example,
In your HTML:
<p id="header">hello</p>
In your JS:
var my_selector = "#header";
$(my_selector).html('wow');

Catching ⌘+1 in Safari JavaScript

I'm writing a Safari 6 extension which adds a few keyboard shortcuts. One of the things I'd like to do is catch when the user presses ⌘+1 ⌘+2, etc. Right now, Safari seems to not fire any event when this happens. Here's a Fiddle: http://jsfiddle.net/Xe9YQ/ to show the code, and here's the JS:
$( 'body' ).bind( 'keypress', function ( event ) {
var modifier = '';
if ( event.metaKey ) {
modifier += 'cmd-';
}
if ( event.ctrlKey ) {
modifier += 'ctrl-';
}
if ( event.altKey ) {
modifier += 'alt-';
}
if ( event.shiftKey ) {
modifier += 'shift-';
}
$( 'body' ).html( modifier + event.keyCode );
});
If you try "⌘+j", "⌘+t", or even "⌘+0" and "⌘+shift+5" you'll see correct output. From this, it seems that it's not a problem of overriding browser shortcuts, and not a problem of using the numerical row.
Is anyone familiar with this problem? Is this a known bug? I'd appreciate any advice.

You have to use the keydown event in combination with preventDefault(), because ⌘ combinations may have bindings already (in Chrome, for example ⌘ + 1 switches to the first tab).
$( 'body' ).bind( 'keydown', function ( event ) {
var modifier = '';
if ( event.metaKey ) {
event.preventDefault();
modifier += 'cmd-';
}
if ( event.ctrlKey ) {
modifier += 'ctrl-';
}
if ( event.altKey ) {
modifier += 'alt-';
}
if ( event.shiftKey ) {
modifier += 'shift-';
}
$( 'body' ).html( modifier + event.keyCode );
});

Use jQuery to switch between left and right text-alignment, depending on the language

I have input boxes and textareas that need to be toggled between left-alignment and right-alignment (depending on the user's language, the direction would be different). How can I do this with jQuery?

As I don't know the key code for all Persian letters, I had to do it like this:
var str = $('#item').val(); //this is your text box
var firstChar = str.substr(0,1);
var characters = ['ا','ب','پ','ت','س','ج','چ','ح','خ','د','ذ','ر','ز','ژ','س','ش','ص','ض','ط','ظ','ع','غ','ف','ق','ک','گ','ل','م','ن','و','ه','ی'];
function checkPersian() {
var result = false;
for (i = 0 ; i<32 ; i++) {
if (characters[i] == firstChar) {
result = true;
}
}
return result;
}
if (checkPersian()) {
$('#item').css('direction','rtl');
} else {
$('#item').css('direction','ltr');
}

Here I have completely overhauled Mohammad's script but it's limited to its purpose: scanning if the first letter the user types in is Persian and changing the inputs direction according.
Here it is: jsfiddle.net/uPH7N/4

You can use dir="auto" attribute in modern browsers: Live Demo
<input type="text" dir="auto"><br>
Also you can do it by jQuery like this: Live Demo
$('input, textarea').keyup(function() {
$(this).val().charAt(0).charCodeAt(0) < 200 ? $(this).css('direction','ltr') : $(this).css('direction','rtl');
});

Toggle the elements' css with jquery (assuming the relevant inputs/textareas all have the class textdirectionBoxes):
$('.textdirectionBoxes').css('direction', 'rtl');
and
$('.textdirectionBoxes').css('direction', 'ltr');

What about
jQuery( document ).ready( function(){
var language = navigator.userLanguage || navigator.language;
if( jQuery.inArray( language, ['ar', 'he', 'ur'] ) != -1 )
jQuery( 'input[type="text"], input[type="password"], textarea' ).css( 'direction', 'rtl' );
});
?
edit: fixed little code-bug

function isUnicode(str) {
return (str.charCodeAt(str.length-1) > 255) ? true : false;
}
$('input[type=text]').each(function() {
$(this).keyup(function(e) {
$(this).css('direction',
isUnicode($(this).val()) ? 'rtl' : 'ltr'
);
});
});

Check First Char In String

I have input-box. I'm looking for a way to fire-up alert() if first character of given string is equal to '/'...
var scream = $( '#screameria input' ).val();
if ( scream.charAt( 0 ) == '/' ) {
alert( 'Boom!' );
}
It's my code at the moment. It doesn't work and I think that it's because that browser doesn't know when to check that string... I need that alert whenever user inputs '/' as first character.

Try this out:
$( '#screameria input' ).keyup(function(){ //when a user types in input box
var scream = this.value;
if ( scream.charAt( 0 ) == '/' ) {
alert( 'Boom!' );
}
})
Fiddle: http://jsfiddle.net/maniator/FewgY/

You need to add a keypress (or similar) handler to tell the browser to run your function whenever a key is pressed on that input field:
var input = $('#screameria input');
input.keypress(function() {
var val = this.value;
if (val && val.charAt(0) == '/') {
alert('Boom!');
}
});