Best practices for using useMemo with TypeScript in React Native

Understanding useMemo in React Native with TypeScript

useMemo memorizes the result of a computation and only recomputes it when one or more dependencies change. It helps to avoid costly recalculations on every component render. When used with TypeScript, it also enforces type safety on the computed values and dependencies.

The syntax typically looks like this in React Native with TypeScript:

tsx
import React, { useMemo } from 'react';

function Component({ data }: { data: number[] }) {
  const computedValue = useMemo(() => {
    // expensive calculation
    return data.reduce((acc, val) => acc + val, 0);
  }, [data]);

  return {computedValue};
}

This memoizes the outcome of the expensive reduce operation and recalculates only when `data` changes.

Best Practices for Using useMemo with TypeScript

Use useMemo Only for Expensive Calculations

The primary motivation for `useMemo` is to avoid costly recalculations. If the computation is lightweight, applying useMemo adds unnecessary complexity without real benefit. For example, simple arithmetic or string concatenation usually doesn't need memoization.

UseMemo shines when you have:

- Large datasets transformations (e.g., sorting, filtering, mapping)
- Computationally expensive calculations (e.g., statistical computations, aggregation)
- Heavy component rendering logic that involves derived data

Applying `useMemo` in such cases preserves performance and responsiveness.

Avoid Overusing useMemo

Overusing useMemo can lead to hard-to-read, complex code, which negates its performance benefits somewhat in typical React Native apps. Developers often tend to memoize everything, but the rule of thumb is to memoize only when you actually measure or know there is a performance bottleneck.

Excessive memoization can also lead to stale closures if dependencies are not managed properly, which causes bugs.

Keep your component logic straightforward, resorting to `useMemo` for targeted optimizations.

Correctly Specify Dependency Arrays

The dependency array in `useMemo` controls when the memoized value is recalculated. It's important to include every variable or prop used inside the memoized function.

If any dependency is omitted, the memoized value will not reflect the changes correctly, leading to bugs or stale data.

When using TypeScript, dependencies may include:

- Primitive values (string, number, boolean)
- Objects or arrays (ensure stable references or use memoized values)
- Functions (memoize them using useCallback if used as dependencies)

If dependencies change frequently without reason, it may reduce the memoization gain.

Example:

tsx
const memoizedValue = useMemo(() => computeSomething(a, b), [a, b]);

Always use ESLint React hooks rules for automated dependency tracking.

Combine useMemo with React.memo for Component Memoization

In React Native apps, passing memoized values as props to child components that are wrapped in `React.memo` avoids unnecessary re-renders.

For example:

tsx
const Child = React.memo(({ sum }: { sum: number }) => {
  return {sum};
});

function Parent({ data }: { data: number[] }) {
  const sum = useMemo(() => data.reduce((acc, val) => acc + val, 0), [data]);

  return ;
}

Here, `useMemo` ensures `sum` only updates when `data` changes, and `React.memo` prevents `Child` from re-rendering if `sum` is the same.

Memoize Functions with useCallback When Needed

Closely related to `useMemo`, the `useCallback` hook memoizes functions, preventing them from being recreated on every render.

When passing callback functions to child components, memoizing with `useCallback` avoids causing unnecessary child re-renders or effect triggers.

Example:

tsx
const onPress = useCallback(() => {
  console.log('Pressed');
}, []);

Use this pattern alongside `useMemo` for values in typed React Native apps to prevent prop reference changes that lead to wasted renders.

Use TypeScript Types to Ensure Clarity and Safety

TypeScript typing guarantees that the values returned by `useMemo` and dependencies are consistent and clear. Declare types for props, computed values, and callback functions to catch type errors at compile time.

Example:

tsx
const memoizedValue: number = useMemo(() => expensiveCalculation(), [deps]);

Clear typing also aids maintainability and helps other developers understand the codebase more quickly.

Keep Side Effects Out of useMemo

`useMemo` should be used only for synchronously returning computed values. Side effects such as data fetching, subscriptions, or state updates must be handled in `useEffect` or other proper React lifecycles/hooks.

Avoid trying to cause side effects in the memoized computation as React may optimize them away or cause unexpected behavior.

Benchmarks and Profiling Before and After

Since premature optimization can cause complexity, it is best practice to profile your React Native app + TypeScript code with React DevTools or performance profilers before introducing `useMemo`.

Measure the actual impact to confirm that memoization provides gains. Not all operations benefit, and some may perform worse if memoized.

Use profiling to identify performance bottlenecks and apply `useMemo` judiciously during optimization passes.

Handling Complex Dependencies and Reference Values

When dependencies are objects or arrays created inline, they get recreated every render, causing `useMemo` to recompute.

Strategies to handle this include:

- Memoizing dependencies themselves with `useMemo` or `useCallback` to keep stable references.
- Using deep equality memoization libraries cautiously if you need more than reference equality.

Example:

tsx
const stableData = useMemo(() => data, [data]); // if data is coming from outside and stable
const memoizedValue = useMemo(() => compute(stableData), [stableData]);

This avoids false invalidations of memoized values.

Use Case Examples in React Native + TypeScript

Example 1: Memoizing Expensive Calculation

tsx
import React, { useMemo } from 'react';
import { Text } from 'react-native';

type Props = { numbers: number[] };

export default function SumComponent({ numbers }: Props) {
  const expensiveSum = useMemo(() => {
    console.log('Computing sum...'); // only runs when numbers change
    return numbers.reduce((acc, n) => acc + n, 0);
  }, [numbers]);

  return {`Sum: ${expensiveSum}`};
}

Example 2: Preventing Child Re-renders

tsx
import React, { useMemo } from 'react';
import { Text } from 'react-native';

type ChildProps = { count: number };

const Child = React.memo(({ count }: ChildProps) => {
  console.log('Child rerender');
  return {`Count is ${count}`};
});

export default function Parent({ items }: { items: number[] }) {
  const count = useMemo(() => items.length, [items]);

  return ;
}

Example 3: Complex State Calculation

tsx
import React, { useMemo } from 'react';
import { Text } from 'react-native';

type Props = { a: number; b: number };

export default function ComplexCalc({ a, b }: Props) {
  const calculated = useMemo(() => {
    // Some expensive computation combining a and b
    return a * b + Math.sqrt(a + b);
  }, [a, b]);

  return {`Result: ${calculated}`};
}

Common Pitfalls to Avoid

- Using useMemo for cheap operations like simple arithmetic
- Forgetting dependencies in the array or including unnecessary dependencies
- Mutating memoized objects outside useMemo (immutability is key)
- Using useMemo for side effects rather than pure computations
- Overusing useMemo in small components or when no performance problem exists
- Relying on shallow equality alone in complex dependency objects

Summary

In React Native projects using TypeScript, `useMemo` is a crucial optimization tool for memoizing expensive calculations, derived state, and preventing unnecessary renders. Its best use involves:

- Memoizing costly computations only
- Always specifying correct dependencies
- Combining with `React.memo` for efficient prop-driven re-render prevention
- Using TypeScript for strict typing and better maintainability
- Avoiding overuse and premature optimization
- Profiling app performance to target memoization effectively

Following these best practices will lead to well-optimized, clean, and maintainable React Native applications with TypeScript.