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What is a buffer?

A buffer is an area of memory. JavaScript developers are not familiar with this concept, much less than C, C++ or Go developers (or any programmer that uses a system programming language), which interact with memory every day.

It represents a fixed-size chunk of memory (can’t be resized) allocated outside of the V8 JavaScript engine.

You can think of a buffer like an array of integers, which each represent a byte of data.

It is implemented by the Node Buffer class.

Why do we need a buffer?

Buffers were introduced to help developers deal with binary data, in an ecosystem that traditionally only dealt with strings rather than binaries.

Buffers are deeply linked with streams. When a stream processor receives data faster than it can digest, it puts the data in a buffer.

A simple visualization of a buffer is when you are watching a YouTube video and the red line goes beyond your visualization point: you are downloading data faster than you’re viewing it, and your browser buffers it.

How to create a buffer

A buffer is created using the Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.

const buf = Buffer.from('Hey!')

You can also just initialize the buffer passing the size. This creates a 1KB buffer:

const buf = Buffer.alloc(1024)
//or
const buf = Buffer.allocUnsafe(1024)

Using a buffer

Access the content of a buffer

A buffer, being an array of bytes, can be accessed like an array:

const buf = Buffer.from('Hey!')
console.log(buf[0]) //72
console.log(buf[1]) //101
console.log(buf[2]) //121

Those numbers are the Unicode Code that identifies the character in the buffer position (H => 72, e => 101, y => 121)

You can print the full content of the buffer using the toString() method:

console.log(buf.toString())

Notice that if you initialize a buffer with a number that sets its size, you’ll get access to pre-initialized memory that will contain random data, not an empty buffer!

Get the length of a buffer

Use the length property:

const buf = Buffer.from('Hey!')
console.log(buf.length)

Iterate over the contents of a buffer

const buf = Buffer.from('Hey!')
for (const item of buf) {
  console.log(item) //72 101 121 33
}

Changing the content of a buffer

You can write to a buffer a whole string of data by using the write() method:

const buf = Buffer.alloc(4)
buf.write('Hey!')

Just like you can access a buffer with an array syntax, you can also set the contents of the buffer in the same way:

const buf = Buffer.from('Hey!')
buf[1] = 111 //o
console.log(buf.toString()) //Hoy!

Copy a buffer

Copying a buffer is possible using the copy() method:

const buf = Buffer.from('Hey!')
let bufcopy = Buffer.alloc(4) //allocate 4 bytes
buf.copy(bufcopy)

By default you copy the whole buffer. 3 more parameters let you define the starting position, the ending position, and the new buffer length:

const buf = Buffer.from('Hey!')
let bufcopy = Buffer.alloc(2) //allocate 2 bytes
buf.copy(bufcopy, 0, 2, 2)
bufcopy.toString() //'He'

Slice a buffer

If you want to create a partial visualization of a buffer, you can create a slice. A slice is not a copy: the original buffer is still the source of truth. If that changes, your slice changes.

Use the slice() method to create it. The first parameter is the starting position, and you can specify an optional second parameter with the end position:

const buf = Buffer.from('Hey!')
buf.slice(0).toString() //Hey!
const slice = buf.slice(0, 2)
console.log(slice.toString()) //He
buf[1] = 111 //o
console.log(slice.toString())