<pre class="language-javascript"><code class="language-javascript">

/**
 * @param {any} data
 * @param {Object} command
 */
function update(data, newData) {
 const isArray = Array.isArray(data)
 const comandLetter = JSON.stringify(newData).indexOf('$')
 const command = JSON.stringify(newData).substring(comandLetter + 1, comandLetter + 2)

 /* push */
 if (command === 'p' &amp;&amp; isArray) {
 return data.concat(newData['$push'])
 }

 if (isArray) {
 const newResult = [...data]
 const [key] = Object.keys(newData)
 const [value] = Object.values(newData)

 /* set */
 if (command === 's') {
 newResult[key] = value.$set
 
 return newResult
 }

 /* apply */
 newResult[key] = value.$apply(newResult[key]) 
 return newResult
 }

 const copyOfData = JSON.parse(JSON.stringify(data))

 const updateCopy = (original, copy) =&gt; {
 const [key] = Object.keys(copy)
 const [deeperCopy] = Object.values(copy)
 const isMerge = key === '$merge'
 const isPush = key === '$push'

 if (isPush &amp;&amp; !Array.isArray(original[key])) throw Error()

 /* push */
 if (deeperCopy.$push) {
 original[key] = original[key].concat(deeperCopy.$push)
 return
 }
 
 /* set */
 if (deeperCopy.$set) {
 original[key] = deeperCopy.$set
 return
 }

 /* merge */
 if (isMerge) {
 const [property] = Object.keys(deeperCopy)
 const [value] = Object.values(deeperCopy)

 if (original.constructor.name !== 'Object') throw Error()

 original[property] = value
 }

 if (deeperCopy.constructor.name === 'Object' &amp;&amp; !isMerge) {
 return updateCopy(original[key], deeperCopy)
 }
 }

 updateCopy(copyOfData, newData)

 return copyOfData
}

console.log(update({a: [1]}, {a: {$push: [2, 3]}}))

</code></pre>

12 - implement immutability helper

If you use React, you would meet the scenario to copy the state for a slight change. For example, for following state const state = { a: { b: { c: 1 }…