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How to successfully break out of function in javascript?

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);
}
});

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. e.g.
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
}

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

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.

Validating a data input javascript

I have been looking to validate the data input to check whether it is a integer or a string. I looked around and saw some suggestions and typeof suggestions but nothing seems to work.
var nam = prompt("Enter name:")
person.push(nam);
var mk1 = prompt("Enter mark 1:");
var mk1 = parseInt(mk1);
mark1.push(mk1);

If you want to check whether input string is not a number try this:
if (isNaN(parseInt(name, 10)) {
//name is String
} else {
//name is Number
}

use the === operator as below
if (mk1 === parseInt(mk1 , 10))
alert("mk1 is integer")
else
alert("mk1 is not an integer. May be String")
If you don't know that the argument is a number-
function isInt(n){
return Number(n)===n && n%1===0;
}

Try this way to find input type;
if(!isNaN(parseInt(mk1)))
// for integer
else if(!isNaN(parseFloat(mk1)))
//for float
else
// String

When you prompt() the user for data, you always get a string. If you want to check, whether it actually contains just a number, you can try this:
var value = prompt('...'),
num = parseInt(value, 10);
if (num == value) {
// ... it is an integer, use `num`
} else {
// ... it's not an integer (or not *just* an integer), use `value`
}
(or use parseFloat(value) for real numbers).

It's hard to say what are you trying to do really. You seem to declare var mk1 twice, which looks a bit strange. Also, even if parseInt fails (then returns NaN [Not a Number]) you add it to mark1, which is probably not what you want. Have a look at this:
var nam = prompt("Enter name:")
person.push(nam);
var mk1 = prompt("Enter mark 1:");
mk1 = parseInt(mk1);
if (Number.isNaN(mk1) === false) {
mark1.push(mk1);
} else {
alert("mark 1 is not a number");
}

Use this function:
isNaN(parseInt(mk1))
It will return "true" if not a number, and "false" if a number

Integer validation not working as expected

Thanks to some of the answers on this site, I built a function to validate an integer inside a prompt in javascript. I found out how to use isNaN and the result of % in order to meet my needs, but there must be something wrong, because is still not working: This function for validation needs to accept only integers, and as extra bonus, it will also accept a special keyword used for a different purpose later on in the program.
So, previously I had defined:
var value = prompt("Type an integer");
So after that, I made a call for the validation function, and that included three conditions: The validation warning would jump if:
1) The string is not a number
2) The string % 1 is not 0 (means is not an integer)
3) The string is not the special keyword ("extra") which is also valid as input.
The function needs to loop and keep showing the prompt until a valid data is written.
while (isNaN(value) == true && value % 1 != 0 && value != "extra") {
alert("Please, type an integer");
var value = prompt("Type an integer");
}
What am I doing wrong? Thank you so much for any ideas. I know the integer validation has been asked many times here, and here I got a few ideas, but I might be missing something...

You might be complicating things too much... A quick regular expression will do the trick.
while (!/^(\d+|extra)$/i.test(value)) {
...
}

You typed only one equal at
isNaN(value) = true

jsFiddle example
var int = 10;
var str = "10";
var isInt = function(value) {
return (str === 'extra' || !isNaN(parseInt(value, 16)) || /^\d+$/.test(value));
};
var isIntStrict = function(value) {
return (isInt(value) && typeof value !== 'string');
}
console.log('false', isInt('kirk'));
console.log('true', isInt(int));
console.log('true', isInt(str));
console.log('true', 'strict - int', isIntStrict(int));
console.log('false','strict - string', isIntStrict(str));
console.log('false','strict - string', isIntStrict('0x04'));
console.log('true','strict - string', isIntStrict(0x04));

I assume that for your purposes #elclanrs' answer is all you need here, and is the simplest and most straightforward, but just for completeness and dubious laughs, I'm pretty sure that the following would also do what you're looking for:
function isAnIntOrExtra(v) {
if (parseInt(+v) === +v && v !== '') {
return parseInt(+v);
}
else if (v === 'extra') {
return v;
}
else {
return false;
}
}
Fiddle here
These should all pass and return an integer in decimal notation:
'387' returns 387
'-4' returns -4
'0' returns 0
'2.4e3' returns 2400
'0xf4' returns 244
while these should all fail:
'4.5' returns false
'2.4e-3' returns false
'0xgc' returns false
'' returns false
'seven' returns false
And the magic-word 'extra' returns 'extra'
Of course, it'll "fail" miserably with values like '1,345', and will probably roll right over octal notation, treating it as though it were decimal notation (depending on the JavaScript engine?), but it could be tweaked to handle those situations as well, but really, you're better off with the regex.

Why doesnt this Javascript regular expression work

var existing = "";
if(disk.isLinux){
var valinvalid = "/usr" ;
var valinput = /^\/[a-zA-Z]{2,}/ ;
if(!valinput.match(valinvalid)){
return "^/" + existing + "[^/][a-zA-Z]{2,}[^/]$";
}
}
Here im trying to do the following in the first if condition ie. if(disk.isLinux):
1. there should be minimum 3 characters
2. the first character should be /
3. the entire input shouldnt match "/usr". But it can be /us or /usra

If you are just trying to test if it matches, us test on regexp:
/^\/[\w]{2,}/.test("/usr/"); //true
Is this what you are trying to do?

A couple of things:
1) vars should never ever ever be inside if statements
2) String.prototype.match exists RegExp.prototype.match does not
But more importantly, you dont need regEx at all
if (
input.length < 3 ||
input.charAt(0) !== '/' ||
input === '/user'
) {
throw new Error("I'm not happy with the input");
}

try changing your code to use:
var valinput = new RegExp("/^\/[a-zA-Z]{2,}/") ;
if(!valinput.test(valinvalid)){

Javascript, String: Getting a Javascript var as a string

Because using .toString() for null vars doesn't work, and I can't be checking each and every one of these in my particular application.
I know this is a stupidly simple problem with an answer that literally must be staring me in the face right now.

The non-concatenation route is to use the String() constructor:
var str = new String(myVar); // returns string object
var str = String(myVar); // returns string value
Of course, var str = "" + myVar; is shorter and easier. Be aware that all the methods here will transform a variable with a value of null into "null". If you want to get around that, you can use || to set a default when a variable's value is "falsey" like null:
var str = myVar || "";
Just so long as you know that 0 and false would also result in "" here.
https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Objects/String

How about
var str = '' + someVar;

What about
var str = (variable || "");
//or
(variable || "").toString();
Of course you'll get this for false, undefined and so on, too, but it will be an empty string and not "null"

String(null) returns "null", which may cause problems if a form field's value is itself null. How about a simple wrapper function instead?
function toString(v) {
if(v == null || v == undefined) {
return "";
}
return String(v);
}
Only null, undefined, and empty strings should return the empty string. All other falsy values including the integer 0 will return something else. Some tests,
> toString(null)
""
> toString(undefined)
""
> toString(false)
"false"
> toString(0)
"0"
> toString(NaN)
"NaN"
> toString("")
""

How about ... ?
var theVarAsString = "" + oldVar;