shallow

shallow lets you run fast checks on simple data structures. It effectively identifies changes in top-level properties when you're working with data structures that don't have nested objects or arrays within them.

const equal = shallow(a, b)

shallow<T>(a: T, b: T): boolean

shallow returns true when a and b are equal based on a shallow comparison of their top-level properties. Otherwise, it should return false.

When comparing primitive values like strings, numbers, booleans, and BigInts, both Object.is and shallow function return true if the values are the same. This is because primitive values are compared by their actual value rather than by reference.

const stringLeft = 'John Doe'
const stringRight = 'John Doe'

Object.is(stringLeft, stringRight) // -> true
shallow(stringLeft, stringRight) // -> true

const numberLeft = 10
const numberRight = 10

Object.is(numberLeft, numberRight) // -> true
shallow(numberLeft, numberRight) // -> true

const booleanLeft = true
const booleanRight = true

Object.is(booleanLeft, booleanRight) // -> true
shallow(booleanLeft, booleanRight) // -> true

const bigIntLeft = 1n
const bigIntRight = 1n

Object.is(bigIntLeft, bigIntRight) // -> true
shallow(bigIntLeft, bigIntRight) // -> true

When comparing objects, it's important to understand how Object.is and shallow function operate, as they handle comparisons differently.

The shallow function returns true because shallow performs a shallow comparison of the objects. It checks if the top-level properties and their values are the same. In this case, the top-level properties (firstName, lastName, and age) and their values are identical between objectLeft and objectRight, so shallow considers them equal.

const objectLeft = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30,
}
const objectRight = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30,
}

Object.is(objectLeft, objectRight) // -> false
shallow(objectLeft, objectRight) // -> true

When comparing sets, it's important to understand how Object.is and shallow function operate, as they handle comparisons differently.

The shallow function returns true because shallow performs a shallow comparison of the sets. It checks if the top-level properties (in this case, the sets themselves) are the same. Since setLeft and setRight are both instances of the Set object and contain the same elements, shallow considers them equal.

const setLeft = new Set([1, 2, 3])
const setRight = new Set([1, 2, 3])

Object.is(setLeft, setRight) // -> false
shallow(setLeft, setRight) // -> true

When comparing maps, it's important to understand how Object.is and shallow function operate, as they handle comparisons differently.

The shallow returns true because shallow performs a shallow comparison of the maps. It checks if the top-level properties (in this case, the maps themselves) are the same. Since mapLeft and mapRight are both instances of the Map object and contain the same key-value pairs, shallow considers them equal.

const mapLeft = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
])
const mapRight = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
])

Object.is(mapLeft, mapRight) // -> false
shallow(mapLeft, mapRight) // -> true

The shallow function performs a shallow comparison. A shallow comparison checks if the top-level properties of two objects are equal. It does not check nested objects or deeply nested properties. In other words, it only compares the references of the properties.

In the following example, the shallow function returns false because it compares only the top-level properties and their references. The address property in both objects is a nested object, and even though their contents are identical, their references are different. Consequently, shallow sees them as different, resulting in false.

const objectLeft = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30,
  address: {
    street: 'Kulas Light',
    suite: 'Apt. 556',
    city: 'Gwenborough',
    zipcode: '92998-3874',
    geo: {
      lat: '-37.3159',
      lng: '81.1496',
    },
  },
}
const objectRight = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30,
  address: {
    street: 'Kulas Light',
    suite: 'Apt. 556',
    city: 'Gwenborough',
    zipcode: '92998-3874',
    geo: {
      lat: '-37.3159',
      lng: '81.1496',
    },
  },
}

Object.is(objectLeft, objectRight) // -> false
shallow(objectLeft, objectRight) // -> false

If we remove the address property, the shallow comparison would work as expected because all top-level properties would be primitive values or references to the same values:

const objectLeft = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30,
}
const objectRight = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30,
}

Object.is(objectLeft, objectRight) // -> false
shallow(objectLeft, objectRight) // -> true

In this modified example, objectLeft and objectRight have the same top-level properties and primitive values. Since shallow function only compares the top-level properties, it will return true because the primitive values (firstName, lastName, and age) are identical in both objects.

The shallow function checks whether the two objects have the same prototype. If their prototypes are referentially different, shallow will return false. This comparison is done using:

Object.getPrototypeOf(a) === Object.getPrototypeOf(b)

const a = Object.create({}) // -> prototype is `{}`
const b = {} // -> prototype is `Object.prototype`

shallow(a, b) // -> false