memoize-one

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

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. A function memoized with memoize-one simply remembers the last arguments, and if the memoized function is next called with the same arguments then it returns the previous result.

For working with promises, @Kikobeats has built async-memoize-one.

Usage

1// memoize-one uses the default import
2import memoizeOne from 'memoize-one';
3
4function add(a, b) {
5  return a + b;
6}
7const memoizedAdd = memoizeOne(add);
8
9memoizedAdd(1, 2);
10// add function: is called
11// [new value returned: 3]
12
13memoizedAdd(1, 2);
14// add function: not called
15// [cached result is returned: 3]
16
17memoizedAdd(2, 3);
18// add function: is called
19// [new value returned: 5]
20
21memoizedAdd(2, 3);
22// add function: not called
23// [cached result is returned: 5]
24
25memoizedAdd(1, 2);
26// add function: is called
27// [new value returned: 3]
28// 👇
29// While the result of `add(1, 2)` was previously cached
30// `(1, 2)` was not the *latest* arguments (the last call was `(2, 3)`)
31// so the previous cached result of `(1, 3)` was lost

Installation

1# yarn
2yarn add memoize-one
3
4# npm
5npm install memoize-one --save

Function argument equality

By default, we apply our own fast and relatively 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:

1import memoizeOne from 'memoize-one';
2
3// add all numbers provided to the function
4const add = (...args = []) =>
5  args.reduce((current, value) => {
6    return current + value;
7  }, 0);
8const memoizedAdd = memoizeOne(add);

1. there is same amount of arguments

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

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

1memoizedAdd(1, 2);
2// each argument is `===` to the last argument, so cache is used
3memoizedAdd(1, 2);
4// second argument has changed, so add function is called again
5memoizedAdd(1, 3);
6// the first value is not `===` to the previous first value (1 !== 3)
7// so add function is called again
8memoizedAdd(3, 1);

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

1memoizedAdd(NaN);
2// Even though NaN !== NaN these arguments are
3// treated as equal as they are both `NaN`
4memoizedAdd(NaN);

Custom equality function

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

1const memoized = memoizeOne(fn, isEqual);

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

Tip: 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 lodash.isEqual deep equal equality check

lodash.isequal correctly handles deep comparing two arrays

1import memoizeOne from 'memoize-one';
2import isDeepEqual from 'lodash.isequal';
3
4const identity = (x) => x;
5
6const shallowMemoized = memoizeOne(identity);
7const deepMemoized = memoizeOne(identity, isDeepEqual);
8
9const result1 = shallowMemoized({ foo: 'bar' });
10const result2 = shallowMemoized({ foo: 'bar' });
11
12result1 === result2; // false - different object reference
13
14const result3 = deepMemoized({ foo: 'bar' });
15const result4 = deepMemoized({ foo: 'bar' });
16
17result3 === result4; // true - arguments are deep equal

The equality function needs to conform to the EqualityFn type:

1// TFunc is the function being memoized
2type EqualityFn<TFunc extends (...args: any[]) => any> = (
3  newArgs: Parameters<TFunc>,
4  lastArgs: Parameters<TFunc>,
5) => boolean;
6
7// You can import this type
8import type { EqualityFn } from 'memoize-one';

The EqualityFn type allows you to create equality functions that are extremely typesafe. You are welcome to provide your own less type safe equality functions.

Here are some examples of equality functions which are ordered by most type safe, to least type safe:

1// the function we are going to memoize
2function add(first: number, second: number): number {
3  return first + second;
4}
5
6// Some options for our equality function
7// ↑ stronger types
8// ↓ weaker types
9
10// ✅ exact parameters of `add`
11{
12  const isEqual = function (first: Parameters<typeof add>, second: Parameters<typeof add>) {
13    return true;
14  };
15  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
16}
17
18// ✅ tuple of the correct types
19{
20  const isEqual = function (first: [number, number], second: [number, number]) {
21    return true;
22  };
23  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
24}
25
26// ❌ tuple of incorrect types
27{
28  const isEqual = function (first: [number, string], second: [number, number]) {
29    return true;
30  };
31  expectTypeOf<typeof isEqual>().not.toMatchTypeOf<EqualityFn<typeof add>>();
32}
33
34// ✅ array of the correct types
35{
36  const isEqual = function (first: number[], second: number[]) {
37    return true;
38  };
39  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
40}
41
42// ❌ array of incorrect types
43{
44  const isEqual = function (first: string[], second: number[]) {
45    return true;
46  };
47  expectTypeOf<typeof isEqual>().not.toMatchTypeOf<EqualityFn<typeof add>>();
48}
49
50// ✅ tuple of 'unknown'
51{
52  const isEqual = function (first: [unknown, unknown], second: [unknown, unknown]) {
53    return true;
54  };
55  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
56}
57
58// ❌ tuple of 'unknown' of incorrect length
59{
60  const isEqual = function (first: [unknown, unknown, unknown], second: [unknown, unknown]) {
61    return true;
62  };
63  expectTypeOf<typeof isEqual>().not.toMatchTypeOf<EqualityFn<typeof add>>();
64}
65
66// ✅ array of 'unknown'
67{
68  const isEqual = function (first: unknown[], second: unknown[]) {
69    return true;
70  };
71  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
72}
73
74// ✅ spread of 'unknown'
75{
76  const isEqual = function (...first: unknown[]) {
77    return !!first;
78  };
79  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
80}
81
82// ✅ tuple of 'any'
83{
84  const isEqual = function (first: [any, any], second: [any, any]) {
85    return true;
86  };
87  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
88}
89
90// ❌ tuple of 'any' or incorrect size
91{
92  const isEqual = function (first: [any, any, any], second: [any, any]) {
93    return true;
94  };
95  expectTypeOf<typeof isEqual>().not.toMatchTypeOf<EqualityFn<typeof add>>();
96}
97
98// ✅ array of 'any'
99{
100  const isEqual = function (first: any[], second: any[]) {
101    return true;
102  };
103  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
104}
105
106// ✅ two arguments of type any
107{
108  const isEqual = function (first: any, second: any) {
109    return true;
110  };
111  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
112}
113
114// ✅ a single argument of type any
115{
116  const isEqual = function (first: any) {
117    return true;
118  };
119  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
120}
121
122// ✅ spread of any type
123{
124  const isEqual = function (...first: any[]) {
125    return true;
126  };
127  expectTypeOf<typeof isEqual>().toMatchTypeOf<EqualityFn<typeof add>>();
128}

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:

1function getA() {
2  return this.a;
3}
4
5const temp1 = {
6  a: 20,
7};
8const temp2 = {
9  a: 30,
10};
11
12getA.call(temp1); // 20
13getA.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.

Clearing the memoization cache

A .clear() property is added to memoized functions to allow you to clear it's memoization cache.

This is helpful if you want to:

• Release memory
• Allow the result function to be called again without having to change arguments

1import memoizeOne from 'memoize-one';
2
3function add(a: number, b: number): number {
4  return a + b;
5}
6
7const memoizedAdd = memoizeOne(add);
8
9// first call - not memoized
10const first = memoizedAdd(1, 2);
11
12// second call - cache hit (result function not called)
13const second = memoizedAdd(1, 2);
14
15// 👋 clearing memoization cache
16memoizedAdd.clear();
17
18// third call - not memoized (cache was cleared)
19const third = memoizedAdd(1, 2);

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.

1const canThrow = (name: string) => {
2  console.log('called');
3  if (name === 'throw') {
4    throw new Error(name);
5  }
6  return { name };
7};
8
9const memoized = memoizeOne(canThrow);
10
11const value1 = memoized('Alex');
12// result function called: console.log => 'called'
13
14const value2 = memoized('Alex');
15// result function not called (cache hit)
16
17console.log(value1 === value2);
18// console.log => true
19
20try {
21  memoized('throw');
22  // console.log => 'called'
23} catch (e) {
24  firstError = e;
25}
26
27try {
28  memoized('throw');
29  // console.log => 'called'
30  // the result function was called again even though it was called twice
31  // with the 'throw' string
32} catch (e) {
33  secondError = e;
34}
35
36console.log(firstError !== secondError);
37// console.log => true
38
39const value3 = memoized('Alex');
40// result function not called as the original memoization cache has not been busted
41
42console.log(value1 === value3);
43// console.log => true

Function properties

Functions memoized with memoize-one do not preserve any properties on the function object.

This behaviour is correctly reflected in the TypeScript types

1import memoizeOne from 'memoize-one';
2
3function add(a, b) {
4  return a + b;
5}
6add.hello = 'hi';
7
8console.log(typeof add.hello); // string
9
10const memoized = memoizeOne(add);
11
12// hello property on the `add` was not preserved
13console.log(typeof memoized.hello); // undefined

If you feel strongly that memoize-one should preserve function properties, please raise an issue. This decision was made in order to keep memoize-one as light as possible.

For now, the .length property of a function is not preserved on the memoized function

1import memoizeOne from 'memoize-one';
2
3function add(a, b) {
4  return a + b;
5}
6
7console.log(add.length); // 2
8
9const memoized = memoizeOne(add);
10
11console.log(memoized.length); // 0

There is no (great) way to correctly set the .length property of the memoized function while also supporting ie11. Once we remove ie11 support then we plan on setting the .length property of the memoized function to match the original function

→ discussion.

Memoized function type

The resulting function you get back from memoize-one has almost the same type as the function that you are memoizing

1declare type MemoizedFn<TFunc extends (this: any, ...args: any[]) => any> = {
2  clear: () => void;
3  (this: ThisParameterType<TFunc>, ...args: Parameters<TFunc>): ReturnType<TFunc>;
4};

• the same call signature as the function being memoized
• a .clear() function property added
• other function object properties on TFunc as not carried over

You are welcome to use the MemoizedFn generic directly from memoize-one if you like:

1import memoize, { MemoizedFn } from 'memoize-one';
2import isDeepEqual from 'lodash.isequal';
3import { expectTypeOf } from 'expect-type';
4
5// Takes any function: TFunc, and returns a Memoized<TFunc>
6function withDeepEqual<TFunc extends (...args: any[]) => any>(fn: TFunc): MemoizedFn<TFunc> {
7  return memoize(fn, isDeepEqual);
8}
9
10function add(first: number, second: number): number {
11  return first + second;
12}
13
14const memoized = withDeepEqual(add);
15
16expectTypeOf<typeof memoized>().toEqualTypeOf<MemoizedFn<typeof add>>();

In this specific example, this type would have been correctly inferred too

1import memoize, { MemoizedFn } from 'memoize-one';
2import isDeepEqual from 'lodash.isequal';
3import { expectTypeOf } from 'expect-type';
4
5// return type of MemoizedFn<TFunc> is inferred
6function withDeepEqual<TFunc extends (...args: any[]) => any>(fn: TFunc) {
7  return memoize(fn, isDeepEqual);
8}
9
10function add(first: number, second: number): number {
11  return first + second;
12}
13
14const memoized = withDeepEqual(add);
15
16// type test still passes
17expectTypeOf<typeof memoized>().toEqualTypeOf<MemoizedFn<typeof add>>();

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.

The comparisons are not exhaustive and are primarily to show that memoize-one accomplishes remembering the latest invocation really fast. There is variability between runs. 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
• Written in Typescript
• Correct typing for Typescript and flow type systems
• No dependencies