Question

function intFromBytes( x ){
    var val = 0;
    for (var i = 0; i < x.length; ++i) {        
        val += x[i];        
        if (i < x.length-1) {
            val = val << 8;
        }
    }
    return val;
}

function getInt64Bytes( x ){
    var bytes = [];
    var i = 8;
    do {
    bytes[--i] = x & (255);
    x = x>>8;
    } while ( i )
    return bytes;
}

I am trying to convert a javascript number to a byte array and then back to a number. However the above functions produce incorrect output with a very large number.

var array = getInt64Bytes(23423423);    
var value = intFromBytes(array);

console.log(value); //Prints 23423423 - correct

var array = getInt64Bytes(45035996273704);  
var value = intFromBytes(array);

console.log(value); //Prints -1030792152 - incorrect

It is my understanding that javascript floats are 53 bits so it shouldn’t be overflowing? alert(Math.pow(2,53)) works fine.

Answer 1

In JavaScript bit shifts (>>, <<) are always performed on signed, 32-bits integers. This leads to range overflow for large numbers.