convert array value into integer

Question

I am trying to convert bytes to integer number value its working for non negative value but not for negative value. Here is my code:-

var byteArrayToLong = function (byteArray) {
var value = 0; 
for (var i = byteArray.length - 1; i >= 0; i--) {
 value = (value * 256) + byteArray[i];
}
console.log(value);
return value;
};
 byteArrayToLong([158,175,59,0]); //3911582 correct 
 byteArrayToLong([229,93,138,255])//4287258085 incorrect the correct value is (i.e from c# BitConverter.ToInt32() method) -7709211

JSFiddle Demo

• Daniel W.: Blind guess: The C# version is using a signed integer, your code emulates an unsigned integer.
• Umesh Sehta: @DanFromGermany thanks.. what I can do for this in JS?

Answer 1

There is very simple way to convert a value to an integer like below:

var val = 23.234;
console.log(~~val) // ~~ to convert into an integer

You may use this in your conversion...

---

var byteArrayToLong = function (byteArray) {
var value = 0; 
for (var i = byteArray.length - 1; i >= 0; i--) {
 value = (value * 256) + byteArray[i];
}
console.log(~~value);
return ~~value;
};
 byteArrayToLong([158,175,59,0]); //3911582
 byteArrayToLong([229,93,138,255]) //-7709211

Answer 2

C-Link Nepal's solution is very elegant. Here is a more verbose one: You are clearly trying to treat the byte array as a two's complement representation of a number. You have to take into account, that the first bit of the highest byte depicts the sign. Therefore, you should treat the first byte seperatly:

var byteArrayToLong = function (byteArray) {
 var value = 0, firstByteOffset = byteArray.length - 1;
 if (byteArray[firstByteOffset] >= 127) {
 value = byteArray[firstByteOffset] - 256;
 } else {
 value = byteArray[firstByteOffset];
 }
 for (var i = firstByteOffset - 1; i >= 0; i--) {
 value = (value * 256) + byteArray[i];
 }
 console.log(value);
 return value;
};
byteArrayToLong([158,175,59,0]); // 3911582
byteArrayToLong([229,93,138,255]); // -7709211