Efficiently Copying Arrays in JavaScript: Best Practices & Code Examples

Avoid unintended side effects by learning proper techniques for copying arrays in JS

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Working with arrays in JavaScript often involves creating copies — whether shallow or deep — to preserve original state and avoid unwanted mutations. However, novice developers might fall prey to improper copying practices leading to unexpected results due to shared references among objects. This guide explores effective strategies for copying arrays in JavaScript, ensuring best performance and desired behavior.

Shallow Copy vs Deep Copy

Before delving into specific copying techniques, let’s clarify terminology surrounding shallow and deep copies.

A shallow copy creates a new instance of an object but shares references for any nested objects contained therein. Modifications made to those nested objects affect both original and copied instances. On the other hand, a deep copy generates entirely separate instances even at deeper nesting levels. Thus, alterations applied solely impact cloned entities, leaving originals unaffected.

Now, let’s examine popular approaches for copying arrays in JavaScript along with corresponding pros and cons.