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  3. Node.js — Filter Data in Streams

Node.js — Filter Data in Streams

Using streams in Node.js is a powerful mechanism to process data. Especially when working with large data sets where you might want to filter chunks that don’t match a given criteria.

Node.js Series Overview

  1. Increase the Memory Limit for Your Process
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  24. How to Count Words in a File
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  31. Lowercase the First Character of a String in JavaScript or Node.js
  32. Uppercase the First Character of a String in JavaScript or Node.js
  33. Prepend Characters or Words to a String in JavaScript or Node.js
  34. Check if a String is a Number
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  36. Prevent Line Breaks in String Template Literals
  37. How to Implement a Custom `toString` Method (Coming soon)
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  1. Filter Data in Streams
  1. Get Number of Seconds Since Epoch in JavaScript
  2. Get Tomorrow’s Date in JavaScript
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  4. Add Seconds to a Date in Node.js and JavaScript
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  17. How to Format a Date YYYY-MM-DD in JavaScript or Node.js
  18. Convert a Date to UTC in JavaScript or Node.js (Coming soon)
  19. Get Number of Days in a Month (Coming soon)
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  3. for…of vs. for…in Loops
  4. Clone/Copy an Array in JavaScript and Node.js
  5. Get an Array With Unique Values (Delete Duplicates)
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  7. Overview of Promise-Based APIs in Node.js
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  5. Use Numeric Separators for Better Readability
  6. Convert Boolean to Number
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  8. Check if a Value is a Number
  1. How to Merge Objects
  2. How to Check if an Object is Empty in JavaScript or Node.js
  3. How to CamelCase Keys of an Object in JavaScript or Node.js
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  6. Check If a Path Is a Directory
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  1. Using the Styletext Utility for Terminal Text Formatting

Example: Loading Lines From File to Database

Imagine the following example: you want to load a file with rows of data into a database. Your requirement is that each row has a value in the “name” column. If there’s no value, you want to skip it.

your-example-file.csv

name      , department  
-------------------------
Marcus    , streaming  
          , joking        <-- filter this line
Norman    , programming  
Christian , programming

The task is to filter the second row because of the missing name.

How to Filter Data in Streams

Transform streams are a way to filter data in streams. Implement the _transform() function to pass through chunks that match your condition. In case the condition doesn’t match, omit the current chunk and proceed with the next one.

Within the _transform method, you can either use this.push() to pass through data or use the second parameter of the next callback. Here’s a code snippet that uses the next callback:

const { Transform } = require('stream')

class Filter extends Transform {

  constructor() {
    super({
      readableObjectMode: true,
      writableObjectMode: true
    })
  }

  _transform(chunk, encoding, next) {
    if (this.has(chunk.name)) {
      return next(null, chunk)
    }

    next()
  }

  has(value) {
    return !!value
  }
}

module.exports = Filter

Notice that the chunk passes through your transform if it matches the condition. If not, you’re notifying Node.js to process the next chunk by calling next().

The object mode configuration in the constructor ensures that the chunks in _transform are JavaScript objects and not buffers. It also improves the readability of the example 😃

Usage

A way of integrating the filter in your stream is to .pipe your input to the transformer and from there to the destination stream:

const Fs = require('fs')  
const Filter = require('./your-filter-transform')

const transformer = new Filter()  
const input = Fs.createReadStream(source)  
const output = Fs.createReadStream(destination)

input.pipe(transformer).pipe(output)

That’s it! A transform stream is a great way to receive data from an input source, read the chunk, process it, and pass the changed value to the destination.

Enjoy coding & make it rock!

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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