var str = "4039";
var num = parseInt(str, 10);
//or:
var num2 = Number(str);
//or: (when string is empty or haven't any digits return 0 instead NaN)
var num3 = ~~str;
var strWithChars = "abc123def";
var num4 = Number(strWithChars.replace(/[^0-9]/,''));
"abc123def"