obj-comp

This library compares two arbitrarily nested JavaScript objects and returns an array of differences between the objects.

Install

npm install obj-comp

Example

const { compare } = require("obj-comp");
const source = {
    name: {
        first: "Bob",
        last: "Sagat"
    }
};
const destination = {
    name: {
        first: "Robert",
        middle: "James"
    },
    age: 13
};
const result = compare(source, destination);
console.log(result);

Output:

[ { type: 'addition', path: [ 'age' ], value: 13 },
  { type: 'addition', path: [ 'name', 'middle' ], value: 'James' },
  { type: 'deletion', path: [ 'name', 'last' ] },
  { type: 'replacement',
    path: [ 'name', 'first' ],
    oldValue: 'Bob',
    newValue: 'Robert' } ]

Comparisons are performed on nested objects, as deeply nested as you want.

Difference Objects

The return value of the compare function is an array of Difference objects. A Difference object is one of three types of objects: Addition, Deletion, and Replacement. The TypeScript definition of these types is as follows:

export type Difference = Addition | Deletion | Replacement;

type Addition = {
    type: "addition",
    path: string[],
    value: any
};

type Deletion = {
    type: "deletion",
    path: string[],
};

type Replacement = {
    type: "replacement",
    path: string[],
    oldValue: any,
    newValue: any
};

In each of the 3 types, the path property is a string array that gives you a sequence of property accesses to the difference that is described. The type discriminator is one of 3 strings: "addition", "deletion", and "replacement" and it lets you differentiate the 3 types of difference objects.

Examples:

{ type: 'addition', path: [ 'age' ], value: 13 }

Says that the destination object has an additional age property compared to the source, and its value is 13.

{ type: 'addition', path: [ 'name', 'middle' ], value: 'James' }

Says that the object within the destination's name property has an additional middle property compared to the source, and its value is "James".

{ type: 'deletion', path: [ 'name', 'last' ] }

Says that the object within the destination's name property has the last property deleted compared to the nested object within the source object.

{ type: 'replacement',
  path: [ 'name', 'first' ],
  oldValue: 'Bob',
  newValue: 'Robert' }

Says that the object within the destination's name property has its first property's value replaced by the value "Robert", which used to be "Bob".

Known Limitation

This library does not currently support arrays.

Digging Deeper

If you'd like to learn more about this library, feel free to review:

• test.ts
• the source code
• As an example of what you can do with this library, here is an example database migration tool made with obj-comp.