Array handling in JS with functional purity

by Alexis Hope, 29 Jul 2021

A convention has sprung up recently to make pure functions in Javascript. To get purity when dealing with arrays or objects cloning has been encouraged. But the nuance of Javascript and references is tricky. To get it right we need to understand references, mutations, shallow clones and deep clones. Which is quite a mouthful and is easy to trip over when coding.


Know when we’re mutating to use shallow clones and avoid deep clones for internal functions.

More Detailed Explanation

Primitive types in Javascript get passed as value, and composite types (arrays and objects) get passed as a reference. The convention to ensure functional purity encourages cloning the input of a function before use.

function (input) {
  const pureInput = [...input]
  // ...

This does a shallow clone of the array to ensure we’re not going to mutate the value another procedure is using.

In some code bases util libraries like Lodash clondeDeep methods are used.

function (input) {
  const pureInput = _.cloneDeep(input)
  // ...

What’s the difference between the two? We’ll find out.

So Javascript doesn’t give us functional purity, but we mimic it by following conventions.

Passing the value by reference is a runtime optimisation to save copying large blocks of memory for every function call. Because Javascript is a dynamic language it can’t predict how large your array is. So rather than copying it to a function scope, it passes the reference through.

Primitive types (sometimes known as scalar types in C) in Javascript are things like

  1. boolean
  2. string
  3. integer
  4. float
  5. undefined
  6. null

Composite types are things built up with primitive types

  1. Object
  2. Array

When using primitive types it’s not an issue. As the function parameters are duplicated to new variables relative to the function scope.

Side note: primitive types are duplicated except for string which is immutable and passed as a reference also.

const a = 1

function increment(param) {
  param++ // cause a mutation
  return param

increment(a) // returns 2
increment(a) // still returns 2
a // initial value is preserved as 1

Where we run into trouble is with using composite types. For performance reasons natively they are not copied to the function scope but passed as a reference to the original variable.

const b = []

function push(arr, param) {
  arr.push(param) // cause a mutation
  return arr

push(b, 1) // [1]
push(b, 1) // [1, 1]
b // [1, 1]

This causes updates to the value outside of the function scope.

So with this trend to do more Functional Programming in Javascript keeping functions pure. It’s encouraged to duplicate the contents of the input. This solves the issue of unexpectedly mutating a composite value.

const c = []

function pushPure(arr, param) {
  const clone = [...arr]
  clone.push(param) // cause a mutation
  return clone

pushPure(c, 2) // [2]
pushPure(c, 2) // [2]
c // []

Doing a shallow clone before pushing avoids this mutation of values outside of the function scope. This type of cloning when mutating is great. But

Some functions I’ve seen in the wild that should be avoided.

const functionInTheWild = (input) => {
  return [...input].some(i =>

Here we’ve copied an array to check its contents. Which is a wasted duplication. We need to be conscious of how this works and when to employ it. The benefit of duplicating is a more defensive function that is less likely to have adverse effects. However, encouraging duplication unnecessarily is slowing down our application.

One risk to this when using the spread operator … is not doing a deep clone of nested objects. In the next example, both arrays are changed

const budgetData = [
const backup = budgetData

const clonedData = [...backup]

clonedData[0][0] = "abc"


To be safe we need to do a deep clone with a utility like cloneDeep from lodash. This is the safest way to clone data. But comes with even greater performance cost.

const clonedData = _.cloneDeep(backup)

Another issue with this convention to clone is relying on developers to enforce this. Allowing some instances to slip through or encouraging nit picking on PRs debating the purity of functions.

If you really want purity these two tools come to mind.

Eslint FP rule set which includes a no mutation rule.

Immutable js offering non native immutable data structures.

But a small warning both these tools add a fair amount over overhead to a project if you’re not prepared to follow their conventions.

A lot of the time all we’re after is semantically pure functions. In JS without the use of immutable data structures striving for absolute purity is a slippery slope.