How to convert isbn 13 to isbn 10?

Javascript newbie - what is wrong with this statement?

• if (passed ==true && (isbn.value.length != "13" || isbn.value.length != "10")) { alert("The ISBN can only be 10 or 13 digits long."); passed = false; ...show more

• Answer:

  if (passed && ((isbn.value.length = 13 || isbn.value.length = 10) == false)) 1) length is a number, not a string. 2) "boolean value == true" is called a pleonasm - the interpreter has to evaluate passed, then it has to evaluate (passed == true), which is the same value as passed. It's wasted effort. 3) You can drop passed altogether, unless it's usually false - just check for the length (even if passed is usually false, executing the code doesn't cost much): if (!(isbn.value.length = 13 || isbn.value.length = 10)) is only 3 evaluations.