Object Spread in JavaScript Carries Hidden Performance Costs Developers Often Miss

A technical analysis published on DEV Community highlights that JavaScript's object spread syntax, widely used across React, Redux, Vue, and Angular codebases, copies every property of an object on each operation rather than simply updating it. For small objects this overhead is negligible, but for large objects or repeated operations the cost can be significant. A common pattern using spread inside Array.reduce to build lookup maps can silently degrade from O(n) to O(n²) complexity as each iteration copies all previously accumulated data. The article notes that while spread is essential for reference-equality checks in frameworks like React, deeply nested state updates compound the cost by triggering multiple allocations at once. Libraries such as Immer were developed specifically to address this trade-off, allowing developers to write mutation-style code while preserving immutability under the hood.