Parse an Integer Without parseInt

Write toInt(str): turn "1234" into 1234 without parseInt, Number or unary plus (in Python, without int() or float() on the string).

• An optional leading - or + is allowed.
• Anything else that isn't a digit, or a string with no digits, gives NaN (float("nan") in Python).

toInt("1234")   // 1234
toInt("-42")    // -42
toInt("12a")    // NaN

The JavaScript grader switches off parseInt and parseFloat. Hold yourself to the rest of the rules.