Concatenates instead of Addition in Javascript

Question

var tt = gas+0.1
document.write (vartt);

Duplicate

Answer 1

You could make use of Number function too.

var tt = Number(gas) + 0.1;
document.write(tt);

Answer 2

The user entered a string. If you want to do arithmetic with it instead of string concatenation, you must convert to a number. There are many different ways to do that including parseInt(gas, 10), parseFloat(gas), Number(gas) and +gas:

Here's one implementation:

var tt = parseFloat(gas) + 0.1; 
document.write(tt);

Also, your document.write() statement was not correct either. The variable name is just tt, not vartt.

Answer 3

Unless you are using <input type="number" /> for the input, the user provided data will be a string. By default, when you try to add a string + a number it will cast that number to a string. You can do what Видул Петров suggested and add the unary + to gas to force cast it to a number, however if it's still a string that can't be cast to a number (like someone entering in the word 'five' vs '5'), youll get NaN as a result unless you have the proper control over the incoming data.