<pre class="shiki github-dark shaku"><code>/** * @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 }…