What is Memoization in JavaScript? | JavaScript Interview Questions

What You'll Learn

What memoization is
Implementation
Use cases

What is Memoization?

Memoization caches function results to avoid repeated calculations for the same inputs.

Basic Implementation

code.jsJavaScript

function memoize(fn) {
  const cache = new Map();
  return function(...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) {
      return cache.get(key);
    }
    const result = fn.apply(this, args);
    cache.set(key, result);
    return result;
  };
}

// Usage
const expensiveFunc = (n) => {
  console.log('Computing...');
  return n * 2;
};

const memoized = memoize(expensiveFunc);
memoized(5); // Computing... 10
memoized(5); // 10 (cached)
memoized(5); // 10 (cached)

Fibonacci Example

code.jsJavaScript

// Without memoization - O(2^n)
function fib(n) {
  if (n <= 1) return n;
  return fib(n - 1) + fib(n - 2);
}

// With memoization - O(n)
const fibMemo = memoize(function(n) {
  if (n <= 1) return n;
  return fibMemo(n - 1) + fibMemo(n - 2);
});

console.time('fib');
fib(40);      // Very slow
console.timeEnd('fib');

console.time('fibMemo');
fibMemo(40);  // Fast
console.timeEnd('fibMemo');

With WeakMap (for objects)

code.jsJavaScript

function memoizeWithWeakMap(fn) {
  const cache = new WeakMap();
  return function(obj) {
    if (!cache.has(obj)) {
      cache.set(obj, fn(obj));
    }
    return cache.get(obj);
  };
}