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4.1.0 • Public • Published


A memoization library that only caches the result of the most recent arguments.

Build Status npm dependencies Downloads per month min minzip


Cache invalidation is hard:

There are only two hard things in Computer Science: cache invalidation and naming things.

Phil Karlton

So keep things simple and just use a cache size of one!

Unlike other memoization libraries, memoize-one only remembers the latest arguments and result. No need to worry about cache busting mechanisms such as maxAge, maxSize, exclusions and so on which can be prone to memory leaks. memoize-one simply remembers the last arguments, and if the function is next called with the same arguments then it returns the previous result.


Standard usage

import memoizeOne from 'memoize-one';
const add = (a, b) => a + b;
const memoizedAdd = memoizeOne(add);
memoizedAdd(1, 2); // 3
memoizedAdd(1, 2); // 3
// Add function is not executed: previous result is returned
memoizedAdd(2, 3); // 5
// Add function is called to get new value
memoizedAdd(2, 3); // 5
// Add function is not executed: previous result is returned
memoizedAdd(1, 2); // 3
// Add function is called to get new value.
// While this was previously cached,
// it is not the latest so the cached result is lost

Custom equality function

You can also pass in a custom function for checking the equality of two items. The equality function will is used to compare the value of every individual argument.

import memoizeOne from 'memoize-one';
import deepEqual from 'lodash.isEqual';
const identity = x => x;
const defaultMemoization = memoizeOne(identity);
const customMemoization = memoizeOne(identity, deepEqual);
const result1 = defaultMemoization({foo: 'bar'});
const result2 = defaultMemoization({foo: 'bar'});
result1 === result2 // false - difference reference
const result3 = customMemoization({foo: 'bar'});
const result4 = customMemoization({foo: 'bar'});
result3 === result4 // true - arguments are deep equal

Equality function type signature

Here is the expected flow type signature for a custom equality function:

type EqualityFn = (newValue: mixed, oldValue: mixed, index: number) => boolean;

The default equality function is a simple shallow equal check

const simpleIsEqual: EqualityFn = (a: mixed, b: mixed): boolean => a === b;

Equality function with multiple arguments

If the function you want to memoize takes multiple arguments, your custom equality function will be called once for each argument and will be passed each argument's new value and last value.

import memoizeOne from 'memoize-one';
const makeCountObj = (first, second, third) => ({
  first: first.count,
  second: second.count,
  third: third.count,
const areCountPropertiesEqual = (newArg, lastArg) => newArg.count === lastArg.count;
// runs once for first's new and last values, once for second's, etc.
const memoizedMakeCountObj = memoizeOne(makeCountObj, areCountPropertiesEqual);
const result1 = memoizedMakeCountObj(
  {a: '?', count: 1},
  {a: '$', count: 2},
  {a: '#', count: 3}
const result2 = memoizedMakeCountObj(
  {b: null, count: 1},
  {b: null, count: 2},
  {b: null, count: 3}
result1 === result2; // true - same reference

Equality function index

For each call of the equality function you are provided with the index of the argument.

import memoizeOne from 'memoize-one';
import deepEqual from 'lodash.isEqual';
const myEqualFn = (newArg, lastArg, index) => {
  // use deep equal for first arg
  if(index === 0) {
    return deepEqual(newArg, lastArg);
  // use shallow equal for all other arguments
  return newArg === lastArg;
const fn = (...args) => {
  console.log('called with', ...args);
const memoized = memoizeOne(fn, myEqualFn);
memoized({hello: 'world'}, 5);
// console.log('called with', {hello: 'world'}, 5);
memoized({hello: 'world'}, 5);
// no call to console.log


# yarn 
yarn add memoize-one
# npm 
npm install memoize-one --save

Module usage

ES6 module

import memoizeOne from 'memoize-one';


If you are in a CommonJS environment (eg Node), then you will need to add .default to your import:

const memoizeOne = require('memoize-one').default;


memoize-one correctly respects this control

This library takes special care to maintain, and allow control over the the this context for both the original function being memoized as well as the returned memoized function. Both the original function and the memoized function's this context respect all the this controlling techniques:

  • new bindings (new)
  • explicit binding (call, apply, bind);
  • implicit binding (call site:;
  • default binding (window or undefined in strict mode);
  • fat arrow binding (binding to lexical this)
  • ignored this (pass null as this to explicit binding)

Changes to this is considered an argument change

Changes to the running context (this) of a function can result in the function returning a different value even though its arguments have stayed the same:

function getA() {
  return this.a;
const temp1 = {
  a: 20,
const temp2 = {
  a: 30,
}; // 20; // 30

Therefore, in order to prevent against unexpected results, memoize-one takes into account the current execution context (this) of the memoized function. If this is different to the previous invocation then it is considered a change in argument. further discussion.

Generally this will be of no impact if you are not explicity controlling the this context of functions you want to memoize with explicit binding or implicit binding. memoize-One will detect when you are manipulating this and will then consider the this context as an argument. If this changes, it will re-execute the original function even if the arguments have not changed.

When your result function throws

There is no caching when your result function throws

If your result function throws then the memoized function will also throw. The throw will not break the memoized functions existing argument cache. It means the memoized function will pretend like it was never called with arguments that made it throw.

const canThrow = (name: string) => {
  if(name === 'throw') {
    throw new Error(name);
  return { name };
const memoized = memoizeOne(canThrow);
const value1 = memoized('Alex');
// console.log => 'called'
const value2 = memoized('Alex');
// result function not called
console.log(value1 === value2);
// console.log => true
try {
  // console.log => 'called'
} catch(e) {
  firstError = e;
try {
  // console.log => 'called'
  // the result function was called again even though it was called twice
  // with the 'throw' string
} catch(e) {
  secondError = e;
console.log(firstError !== secondError);
const value3 = memoized('Alex');
// result function not called as the original memoization cache has not been busted
console.log(value1 === value3);
// console.log => true

Performance 🚀


memoize-one is super lightweight at min minified and minzip gzipped. (1KB = 1,024 Bytes)

Extremely fast

memoize-one performs better or on par with than other popular memoization libraries for the purpose of remembering the latest invocation.


The comparisions are not exhaustive and are primiarly to show that memoize-one accomplishes remembering the latest invocation really fast. The benchmarks do not take into account the differences in feature sets, library sizes, parse time, and so on.

Code health 👍


npm i memoize-one

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