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Node.js Stream readable.unshift() Method

Last Updated : 11 Oct, 2021
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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.

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nidhi1352singh
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