Object mode

Learn the difference between stream object mode and binary mode.

What is object mode?

Streams default to binary mode: Buffer and string chunks flow. When object mode is on, each chunk is an object; this is useful for streaming numbers, arrays or records.

A chunkType(mode) function models the split: if mode === 'object' it returns 'object chunk', otherwise 'binary chunk'. In object mode highWaterMark counts objects, not bytes; this is why values like 16 objects are used. Object mode is turned on with new Readable({ objectMode: true }). Here we pin the split down with a single function; in a real stream the objectMode option makes the call.

The chunk type decision

A chunk's type depends on the stream's mode. In object mode a chunk is an object; in binary mode it is a Buffer or string. This split shapes how you handle data in _read and _write implementations.

Example: chunkType('object') returns 'object chunk'; chunkType('binary') returns 'binary chunk'. In object mode stream.read() returns an object; in binary mode it returns a Buffer. This simple function makes the link between mode and chunk type explicit. Later, when you write a Transform stream, setting objectMode is how you enforce this decision.

Exercise Write the chunk type function

Write function chunkType(mode). Start with if (mode === 'object') return 'object chunk';. End with return 'binary chunk';. Print console.log(chunkType('object')) and console.log(chunkType('binary')).

Show example solution

function chunkType(mode) {
  if (mode === 'object') return 'object chunk';
  return 'binary chunk';
}
console.log(chunkType('object'));
console.log(chunkType('binary'));

Test yourself

Match each piece to its role

Object mode

Carries objects
Binary mode

Carries Buffers
objectMode: true

Option
highWaterMark

Threshold
What does chunkType('object') return?
```
function chunkType(m) {
  if (m === 'object') return 'object chunk';
  return 'binary chunk';
}
chunkType('object')
```
- binary chunk
- Buffer
- object chunk
- undefined
Show answer

object chunk — mode is 'object', so it returns 'object chunk'.

Complete the condition

if (mode === '____') return 'object chunk';

Show answer

if (mode === 'object') return 'object chunk';

Next lesson: Pipeline

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