Instead of adding together, it concatenates [duplicate]

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First of all, your ternary operator seems to be quite repetetive. There is a way to make it shorter:

totalPrice = salesPrice + ( salesPrice <= 25 ? minShipping : maxShipping );

Then, it is a good habit to declare your variables before you use them using var keyword. In this particular case the advice is useless, since the script will be executed in the global scope, so all your variables will end up being global.

Your version of script concatenates the result since the return type of window.prompt is String. And when the one tries to add a String to a Number, the latter just gets converted into a String and the concatenation occurs.

You can explicitly convert String to Number by using built-in parseInt (or parseFloat) functions or by adding a plus sign.

<script type="text/javascript">
/*<CDATA[[*/
var salesPrice, minShipping, maxShipping, totalPrice;

salesPrice  = +window.prompt( 'Please Enter Purchase Price?', '' ),
minShipping = salesPrice * 1.5 / 100;
maxShipping = salesPrice * 10  / 100;

totalPrice  = salesPrice + ( salesPrice <= 25 ? minShipping : maxShipping );

alert( totalPrice );
/*]]>*/
</script>

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