memoize-one

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

Also async version.

Rationale

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.

Usage

// memoize-one uses the default import
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

Installation

# yarn
yarn add memoize-one

# npm
npm install memoize-one --save

Function argument equality

By default, we apply our own fast and naive equality function to determine whether the arguments provided to your function are equal. You can see the full code here: are-inputs-equal.ts.

(By default) function arguments are considered equal if:

1. there is same amount of arguments
2. each new argument has strict equality (===) with the previous argument
3. [special case] if two arguments are not === and they are both NaN then the two arguments are treated as equal

What this looks like in practice:

import memoizeOne from 'memoize-one';

// add all numbers provided to the function
const add = (...args = []) =>
  args.reduce((current, value) => {
    return current + value;
  }, 0);
const memoizedAdd = memoizeOne(add);

1. there is same amount of arguments

memoizedAdd(1, 2);
// the amount of arguments has changed, so underlying add function is called
memoizedAdd(1, 2, 3);

2. new arguments have strict equality (===) with the previous argument

memoizedAdd(1, 2);
// each argument is `===` to the last argument, so cache is used
memoizedAdd(1, 2);
// second argument has changed, so add function is called again
memoizedAdd(1, 3);
// the first value is not `===` to the previous first value (1 !== 3)
// so add function is called again
memoizedAdd(3, 1);

3. [special case] if the arguments are not === and they are both NaN then the argument is treated as equal

memoizedAdd(NaN);
// Even though NaN !== NaN these arguments are treated as equal
memoizedAdd(NaN);

Custom equality function

You can also pass in a custom function for checking the equality of two sets of arguments

const memoized = memoizeOne(fn, isEqual);

The equality function needs to conform to this type:

type EqualityFn = (newArgs: any[], lastArgs: any[]) => boolean;

// You can import this type from memoize-one if you like

// typescript
import { EqualityFn } from 'memoize-one';

// flow
import type { EqualityFn } from 'memoize-one';

An equality function should return true if the arguments are equal. If true is returned then the wrapped function will not be called.

A custom equality function needs to compare Arrays. The newArgs array will be a new reference every time so a simple newArgs === lastArgs will always return false.

Equality functions are not called if the this context of the function has changed (see below).

Here is an example that uses a dequal deep equal equality check

dequal correctly handles deep comparing two arrays

import memoizeOne from 'memoize-one';
import { dequal as isDeepEqual } from 'dequal';

const identity = (x) => x;

const shallowMemoized = memoizeOne(identity);
const deepMemoized = memoizeOne(identity, isDeepEqual);

const result1 = shallowMemoized({ foo: 'bar' });
const result2 = shallowMemoized({ foo: 'bar' });

result1 === result2; // false - different object reference

const result3 = deepMemoized({ foo: 'bar' });
const result4 = deepMemoized({ foo: 'bar' });

result3 === result4; // true - arguments are deep equal

this

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: obj.foo());
• 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,
};

getA.call(temp1); // 20
getA.call(temp2); // 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) => {
  console.log('called');
  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 {
  memoized('throw');
  // console.log => 'called'
} catch (e) {
  firstError = e;
}

try {
  memoized('throw');
  // 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 🚀

Tiny

memoize-one is super lightweight at minified and 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.

Results

• simple arguments
• complex arguments

The comparisons are not exhaustive and are primarily 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 👍

• Tested with all built in JavaScript types.
• 100% code coverage
• Continuous integration to run tests and type checks.
• Written in Typescript
• Correct typing for Typescript and flow type systems
• No dependencies