Node.js中的缓冲区问题

Question

I have one small issue while using buffers in Node.js

I have defined my constant buffer like this.

var commands = {
    BufferOne : new Buffer([0XCA, 0X11, 0X00, 0X00, 0X60, 0X01, 
              0X00, 0X01, 0X08, 0X07, 0X6D, 0X00, 0X00, 0X00,
              0X00, 0X00, 0X4F, 0X41, 0X30, 0X30, 0X00])
}

Now later when I receive any specific event in program i want to update 14, 15 from that.

Let's say for example in one of the function i will create local variable

var bufferCopy = commands.BufferOne;
bufferCopy[14] = "0X02";
bufferCopy[15] = "0X00";

Then I want to use that bufferCopy to send to the serial port but my functionality doesn't work.

Now same way from above if I do like this

var bufferCopy = new Buffer([0XCA, 0X11, 0X00, 0X00, 0X60, 0X01, 0X00,
   0X01, 0X08, 0X07, 0X6D, 0X00, 0X00, 0X00, 0X00, 0X00, 0X4F, 0X41,
   0X30, 0X30, 0X00]);
bufferCopy[14] = "0X02";
bufferCopy[15] = "0X00";

Then it works fine. I can get the result on the serialport.

So I am not able to understand why i can't create any local variable from constant declared above.

The reason i want to use constant is because i want to move those all constant to some server so I can send those commands from server instead of keeping in Node Program itself.

Answer 1

When you assign a Buffer like that ( var bufferCopy = commands.BufferOne; ), it's creating a reference (just like all object assignments in JavaScript) to the original Buffer, so you are actually modifying commands.BufferOne instead of a copy of commands.BufferOne .

If you want a copy, you will have to explicitly create a copy, like:

var bufferCopy = new Buffer(commands.BufferOne);
bufferCopy[14] = "0X02";
bufferCopy[15] = "0X00";

Then only bufferCopy will be modified and not also commands.BufferOne .