Deep Copy vs Shallow Copy in JavaScript

Aug 6, 2019

When you clone an object in JavaScript, you can either create a deep copy or a shallow copy. The difference is that a deep copy, like Lodash's cloneDeep() function, recursively clones all nested objects.

For example, the Object.assign() function lets you shallow clone an object.

const obj = { answer: 42 };
// The `Object.assign()` function is a common way to shallow clone an object
const copy = Object.assign({}, obj);

++copy.answer; // 43
obj.answer; // 42, did not change because `copy` is a copy of `obj`

Shallow cloning does not copy nested objects. So if obj contains an object name, Object.assign() will keep a reference to the original copy of name rather than creating a copy of name.

const obj = { name: { first: 'Jean-Luc', last: 'Picard' } };
const copy = Object.assign({}, obj); = 'Johnny';; // 'Johnny', `name` was **not** cloned

Generally, to deep clone an object, you need a library like Lodash. There is an alternative: serializing the object to JSON using JSON.stringify() and then parsing it back using JSON.parse().

const obj = { name: { first: 'Jean-Luc', last: 'Picard' } };
const copy = JSON.parse(JSON.stringify(obj)); = 'Johnny';; // 'Jean-Luc'

However, this approach only works well if your object only contains strings, numbers, booleans, objects, and arrays. For example, if your object has a property date that is a JavaScript date, the cloned object's date property will be a string, because that's how JSON.stringify() serializes dates.

const obj = { date: new Date('2019-06-01') };
const copy = JSON.parse(JSON.stringify(obj)); instanceof Date; // true instanceof Date; // false, `date` is a string

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