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Delta Format

This page intends to be a reference for JSON format used to represent deltas (i.e. the output of jsondiffpatch.diff).

This format was created with a balance between readability and low footprint in mind.

  • when diffing 2 objects, the delta will reflect the same object structure (common part on both sides)
  • to represent changed parts, arrays and magic numbers are used to keep a low footprint (i.e. you won't see verbosity like "type": "added")
  • keep it pure JSON serializable

A great way to understand this format is using the "Annotated JSON" option in the Live Demo, and try the different left/right examples, or edit left/right JSON to see the annotated delta update as your type.

Here's a complete reference of this format.


a value was added, i.e. it was undefined and now has a value.

delta = [ newValue ]


a value was replaced by another value

delta = [ oldValue, newValue ]


a value was deleted, i.e. it had a value and is now undefined

delta = [ oldValue, 0, 0 ]

Object with inner changes

value is an object, and there are nested changes inside its properties

delta = {
  property1: innerDelta1,
  property2: innerDelta2,
  property5: innerDelta5

Note: only properties with inner deltas are included

Here's an example combining what we have:

delta = {
  property1: [ newValue1 ], // obj[property1] = newValue1
  property2: [ oldValue2, newValue2 ], // obj[property2] = newValue2 (and previous value was oldValue2)
  property5: [ oldValue5, 0, 0 ] // delete obj[property5] (and previous value was oldValue5)

Array with inner changes

value is an array, and there are nested changes inside its items

delta = {
  _t: 'a',
  index1: innerDelta1,
  index2: innerDelta2,
  index5: innerDelta5

Note: only indices with inner deltas are included

Note: _t: 'a', indicates this applies to an array, when patching if a regular object (or a value type) is found, an error will be thrown

Index Notation

Indices on array deltas can be expressed in two ways:

  • number: refers to the index in the final (right) state of the array, this is used to indicate items inserted.
  • underscore + number: refers to the index in the original (left) state of the array, this is used to indicate items removed, or moved.

Array Moves

an item was moved to a different position in the same array

delta = [ '', destinationIndex, 3]

Note: '' represents the moved item value, suppresed by default

Note: 3 is the magical number that indicates "array move"

Text Diffs

If two strings are compared and they are different, you will see as you expect:

delta = [ "some text", "some text modified" ]

But if both strings are long enough, a text diffing algorithm will be used to efficiently detect changes in parts of the text.

You can modify the minimum length with:

var customDiffPatch = jsondiffpatch.create({
  textDiff: {
    minLength: 60 // default value

And the delta will look like this:

delta = [ unidiff, 0, 2 ]

Note: 2 is the magical number that indicates "text diff"

Note: unidiff is actually a character-based variation of Unidiff format that is explained here