How can I get an alert if a specific url substring is present and nothing if null - substring

Question

function getCode() {
if (window.location.href.indexOf("?discount=")) {
var url = (document.URL);
var id = url.substring(url.lastIndexOf('=') + 1);
window.alert(id);
return true;
} else {
return false;
}
}
Purpose: When people go to our "Service Request" page using a QR code that has a substring of ?discount=1234. I have been testing by creating an alert box with the discount code showing. Eventually I want to be able to populate that "1234" automatically into a "Discount Code:" text field on page load.
The above is a mixture of a few suggestions when I researched it.
Result: Going to example.com/serviceRequest.html?discount=1234 gives me the appropriate alert "1234", as I want... Going to example.com/serviceRequest.html gives me the alert http://example.com/serviceRequest.html, but I don't want anything to happen if "?discount=" is null.
Any suggestions?

Answer 1

indexOf returns -1 if the search pattern doesn't exist. In JavaScript, anything not a 0 or false or undefined is considered true.
So your line of:
if(window.location.href.indexOf("?discount=")) {
Would better search as:
if(window.location.href.indexOf("?discount=") > -1) {

Answer 2

Try changing your if-statement to:
if(window.location.href.indexOf("?discount=") != -1)

Answer 3

Look up the documentation for ".indexOf". It returns -1 for not found and >= 0 if it is found.
...indexOf("?discount=") >= 0

Answer 4

substring and indexOf return -1 if the text is not found, so you can test for this. E.g.
function getCode() {
if(window.location.href.indexOf("?discount=") != -1) {
var url = (document.URL);
var id = url.substring(url.lastIndexOf('=') + 1);
window.alert(id);
return true;
}
else {
return false;
}
}

Answer 5

You just need to test the indexOf value:
function getCode() {
if (window.location.href.indexOf("?discount=") !== -1) {
var url = (document.URL);
var id = url.substring(url.lastIndexOf('=') + 1);
window.alert(id);
return true;
}
else {
return false;
}
}

Answer 6

So the quick and dirty answer would be
var discount = window.location.search.split("?discount=")[1];
alert(discount);
But this doesn't take into account the occurence of other query string parameters.
You'll really want to parse all the query parameters into a hash map.
This article does a good job of showing you a native and jQuery version.
http://jquery-howto.blogspot.com/2009/09/get-url-parameters-values-with-jquery.html