Transform stream
- Node.js course
- Intermediate
- Lesson 11/116
- 5 min
Learn to transform data with a Transform stream.
Transform stream
A Transform stream both reads and writes: it takes an input chunk, transforms it and passes the output chunk. It is created with new Transform({ transform(chunk, encoding, callback) { ... } }).
The transform method is called for each input chunk. You pass the transformed data with callback(null, output); the first argument is the error (null = no error). Example: const upper = new Transform({ transform(chunk, enc, cb) { cb(null, chunk.toString().toUpperCase()); } });. The call upper.write("hello"); upper.end(); produces HELLO. You catch the output with upper.on("data", (c) => console.log(c.toString())). Transform is the middle link of a pipe chain.
Transform and pipe
Transform streams are the middle link of a pipe chain. With source.pipe(transformer).pipe(target) data flows from the source, is transformed and written to the target. This is the idiomatic way to process data fluently.
Example: const upper = new Transform({ transform(c, e, cb) { cb(null, c.toString().toUpperCase()); } }); source.pipe(upper).pipe(target);. Each chunk is uppercased as it flows to the target. It can also be used alone: upper.write("a"); upper.on("data", (c) => console.log(c.toString())); upper.end();. This prints A. Transform is a powerful tool combining Readable and Writable.
Practice Write a Transform
Create a Transform named upper: its transform(chunk, enc, cb) body is cb(null, chunk.toString().toUpperCase()). Listen for the "data" event: for each c print "Transformed: " + c.toString(). Call upper.write("hello") and upper.end().
Starter code
// Import Transform and create an upper instance
// write here
// Listen for the "data" event
// write here
// Write "hello" and call end()
// write hereSample solution
const { Transform } = require("stream");
const upper = new Transform({
transform(chunk, enc, cb) { cb(null, chunk.toString().toUpperCase()); }
});
upper.on("data", (c) => console.log("Transformed: " + c.toString()));
upper.write("hello");
upper.end();Expected output
Dönüştü: MERHABA
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the transform body
cb(null, chunk.toString().to____Case());cb(null, chunk.toString().toUpperCase());Explanation: toUpperCase uppercases letters.
2 What is the core job of a Transform stream?
new Transform({ transform(c, e, cb) { cb(null, c.toString().toUpperCase()); } })- Only reads data
- Writes data to memory
- Transforms the incoming chunk and passes on the outgoing one Correct answer
- Throws an error
Explanation: Transform takes input, transforms it and passes output.
3 Match each piece to its role
| Term | Meaning |
|---|---|
transform(chunk, ...) |
Processes the input chunk |
cb(null, output) |
Passes the transformed data |
toUpperCase() |
Uppercases letters |
.pipe(transform) |
Adds a transformer to the chain |