Example usage of the generic-diff module

00-output

 one
-two
+twos
 three

a.txt

one
two
three

b.txt

one
twos
three

diff.js

#!/usr/bin/env node
'use strict'

var diff = require('generic-diff')
var fs = require('fs')
var os = require('os')

// Takes an array of edits containing “one or more” items each, and turns
// it into an array of edits with exactly one item per edit.
function flatten (changes) {
  return changes.reduce(function (arr, edit) {
    return arr.concat(edit.items.map(function (item) {
      return {
        item: item,
        added: edit.added,
        removed: edit.removed
      }
    }))
  }, [])
}

// Read two files passed in on the command line, splitting them at
// newlines.
var before = process.argv[2]
before = fs.readFileSync(before, { encoding: 'utf-8' })
before = before.split(/\r?\n/)

var after = process.argv[3]
after = fs.readFileSync(after, { encoding: 'utf-8' })
after = after.split(/\r?\n/)

// Perform the diff. Prepend added lines with a '+', removed lines with
// a '-', and unchanged lines with a space.
var changes = flatten(diff(before, after))
changes = changes.map(function (edit) {
  var pre = ' '

  if (edit.added) {
    pre = '+'
  } else if (edit.removed) {
    pre = '-'
  }

  return pre + edit.item
})

// That’s it!
console.log(changes.join(os.EOL))