JavaScript Validate a Valid Date formatted as "MM/DD/YYYY"

Validates the input string is a valid date formatted as "MM/DD/YYYY"
function Isvaliddate (datestring) {
First check for the pattern
if (!/^\d{1,2}\/\d{1,2}\/\d{4}$/.test (datestring)) {
return false;
}
Parse the date parts to integers
var parts = datestring.split ("/");
var day = parseint (parts[1], 10);
var month = parseint (Parts[0], 10);
var year = parseint (parts[2], 10);

Check the ranges of month and year
if (Year < + | | year > | | month = = 0 | | month > 12) {
return false;
}
var monthlength = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

Adjust for leap years
If (year% 400 = = 0 | | (Year% + = 0 && Year% 4 = = 0))
MONTHLENGTH[1] = 29;

Check the range of the day
Return Day > 0 && Day <= monthlength[month-1];
};

JavaScript Validate a Valid Date formatted as "MM/DD/YYYY"