Performance Considerations

When working with arrays, performance can vary depending on the operations you perform. Understanding how JavaScript engines optimize array operations can help you write efficient code.

Key Points

• Preallocate length when you know the size to avoid dynamic resizing costs.
• Mutator methods modify in place and are usually faster than creating new arrays.
• Avoid sparse arrays – they are slower and have inconsistent behavior.
• Use for loops instead of forEach when performance is critical (though modern engines optimize both well).
• Avoid delete – it leaves holes; use splice or set to undefined.
• Typed arrays (Uint8Array, etc.) offer better performance for numeric data.
• slice and concat create shallow copies; be mindful of memory usage.

Code Example

// Preallocation
const size = 1000000;
const arr = new Array(size);
for (let i = 0; i < size; i++) {
  arr[i] = i;
}

// Mutator vs. accessor
const numbers = [1, 2, 3, 4, 5];
// Fast: in-place reverse
numbers.reverse();

// Slower: creates new array
const reversed = numbers.slice().reverse();

// Avoid sparse arrays
const dense = Array.from({ length: 10000 }, (_, i) => i);
// instead of sparse = new Array(10000); then assign individual indices

// Use typed arrays for heavy numeric work
const typed = new Float64Array(10000);
for (let i = 0; i < typed.length; i++) {
  typed[i] = Math.random();
}