Node.js Stream readable.unshift() Method

The readable.unshift() method in a Readable Stream is utilized to push a chunk of data back into the internal buffer. However, when a stream is being fully consumed and it needs to be "un-consumed" then this method is helpful. Syntax:

readable.unshift( chunk, encoding )

Parameters: This method accept two parameters as mentioned above and described below.

chunk: It contains the chunk of data to unshift onto the read queue.
encoding: It holds the encoding type.

Return Value: This method shifts the chunk of data to the internal buffer which can be read-only after calling read method again. Below examples illustrate the use of readable.unshift() method in Node.js: Example 1:

JavaScript

// Node.js program to demonstrate the     
// readable.unshift() method  

// Including fs module
const fs = require('fs');

// Declaring data
var data = '';

// Constructing readable stream
const readable = fs.createReadStream("input.text");

// Instructions for reading data
readable.on('readable', () => {
  let chunk;
 
  // Using while loop and calling
  // read method
  while (null !== (chunk = readable.read())) {
 
    // Displaying the chunk
    console.log(`read: ${chunk}`);
  }
});
console.log("Program ends!!!");

// Calling unshift method
readable.unshift(data);

Output:

Program ends!!!
true
read: GfG

Example 2:

JavaScript

// Node.js program to demonstrate the     
// readable.unshift(chunk[, encoding])
// method  

// Include fs module
const fs = require('fs');
var data = '';

// Create readable stream
const readable = fs.createReadStream("input.text");

// Calling setEncoding method
readable.setEncoding('utf8');

// Instructions to unshift data
readable.on("readable", () => {
  let data = readable.read();
  
  while (data === "GfG") {
  
    readable.unshift(data);
    data = readable.read();
  }
  });

// Displays that program 
// is unshifted
  console.log("Unshifted!!");

Output:

Unshifted!!

Reference: https://nodejs.org/api/stream.html#stream_readable_unshift_chunk_encoding.

Node.js Stream readable.unshift() Method

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