'1' + '2' is concatenation of strings and result is '12'. '12' - 3 is math operation 12 - 3 and result is 9.

You shoud do the intermediate steps and see for yourself.

Try:

a = '1' + '2';
console.log(a);
b = a - 3;
console.log(b);   // prints 9

The + operator is described as for purpose of sum of numeric operands or string concatenation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition

That is because your first two numbers are strings. you would get a correct answer if you used:

1 + 2 - 3

Or if you cannot avoid doing a calculation on a string, convert it to a number first:

parseInt('1') + parseInt('2') - 3