Buffer.concat

Learn to combine buffers with Buffer.concat.

Buffer.concat

Buffer.concat() merges an array of buffers into a single buffer. It is ideal for assembling data that arrives in pieces; for example, you can join chunks coming from a stream.

For example Buffer.concat([Buffer.from("hel"), Buffer.from("lo")]) gives a single five-byte buffer. When the result is converted with .toString() you get hello. You can pass the total length as the second argument; this prevents Node.js from computing it and speeds things up. This lets you easily join small pieces.

The totalLength parameter

You can pass a totalLength as the second argument to Buffer.concat. It is the total byte count of the resulting buffer. If you omit it, Node.js sums the lengths of all items.

For example Buffer.concat([a, b], a.length + b.length) states the total explicitly. For large arrays, computing this yourself saves time, because Node.js does not have to walk every item to sum lengths. However, if you pass a wrong totalLength, the result may be shorter or longer than expected. The safe path is to make sure your sum is correct.

Exercise Concat two buffers and convert to string

Concat Buffer.from("mer") and Buffer.from("haba") with Buffer.concat. Convert to string with .toString() and print with console.log.

Show example solution

console.log(Buffer.concat([Buffer.from("mer"), Buffer.from("haba")]).toString());

Test yourself

What does Buffer.concat take?
```
Buffer.concat([...])
```
- A number
- A single string
- An array of buffers
- A function
Show answer

An array of buffers — concat takes an array of buffers and returns one buffer.

Complete the concat call

const result = Buffer.concat([Buffer.from("mer"), Buffer.____("haba")]);

Show answer

const result = Buffer.concat([Buffer.from("mer"), Buffer.from("haba")]);

Match each piece to its role

Buffer.concat

Joins buffers
array

List of buffers
toString

Converts to string
totalLength

Total byte count

Next lesson: toString and encoding

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