Validate 64-bit integer (Int64) in Javascript

validateInt64.js

function validateInt46(value)
{
    if (!/^[-]?\d+$/.test(value)) return false;
    
    var isNegative = value[0]==='-';
    
    if (value.length < 18 + (isNegative ? 1 : 0)) return true;
    if (value.length > (isNegative ? 20 : 19)) return false;
    if (isNegative) value = value.slice(1);

    var part1 = parseInt(value.slice(0, 10));
    var part2 = parseInt(value.slice(10));
    return part1 <= 9223372036 && part2 <= (isNegative ? 854775808 : 854775807);
}

This is incorrect, btw. Consider 9 000 000 000 999 999 999. Your part1 test should return true immediately if the value is less than your target prefix, and false if it's larger. You only want to check the suffix if the target prefix is equivalent to the max prefix.