A memoization library which only remembers the latest invocation. 

A memoization library which only remembers the latest invocation

alexreardon

memoize-one

export declare type EqualityFn = (newArgs: ReadonlyArray<unknown>, lastArgs: ReadonlyArray<unknown>) => boolean;

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

export default function memoizeOne<ResultFn extends (this: any, ...newArgs: any[]) => ReturnType<ResultFn>>(resultFn: ResultFn, isEqual?: EqualityFn): ResultFn;

  "description": "A memoization library which only remembers the latest invocation",

  "author": "Alex Reardon <alexreardon@gmail.com>",

    "url": "https://github.com/alexreardon/memoize-one.git"

    "@typescript-eslint/eslint-plugin": "^2.0.0",

    "@typescript-eslint/parser": "^2.0.0",

    "rollup-plugin-typescript": "^1.0.1",

    "prettier_target": "src/**/*.{ts,js,jsx,md,json} test/**/*.{ts,js,jsx,md,json}"

    "validate": "yarn lint && yarn typecheck",

    "prettier:check": "yarn prettier --debug-check $npm_package_config_prettier_target",

    "prettier:write": "yarn prettier --write $npm_package_config_prettier_target",

    "lint:eslint": "eslint $npm_package_config_prettier_target",

    "lint": "yarn lint:eslint && yarn prettier:check",

    "build": "yarn build:clean && yarn build:dist && yarn build:typescript && yarn build:flow",

    "build:typescript": "tsc ./src/memoize-one.ts --emitDeclarationOnly --declaration --outDir ./dist",

    "build:flow": "cp src/memoize-one.js.flow dist/memoize-one.cjs.js.flow",

    "perf": "ts-node ./benchmarks/shallow-equal.ts",

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

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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.

// Add function is not executed: previous result is returned

// Add function is called to get new value

// Add function is called to get new value.

// it is not the latest so the cached result is lost

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: readonly unknown[], lastArgs: readonly unknown[]) => boolean;

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

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

import { EqualityFn } from 'memoize-one';

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.

The default equality function is a shallow equal check of all arguments (each argument is compared with `===`). If the `length` of arguments change, then the default equality function makes no shallow equality checks. You are welcome to decide if you want to return `false` if the `length` of the arguments is not equal

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

import isDeepEqual from 'lodash.isequal';

const shallowMemoized = memoizeOne(identity);

const deepMemoized = memoizeOne(identity, isDeepEqual);

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

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

result1 === result2; // false - difference reference

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

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

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

### `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](https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch2.md):

- 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:

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](https://github.com/alexreardon/memoize-one/issues/3).

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](https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch2.md#explicit-binding) or [implicit binding](https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch2.md#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.

> There is no caching when your result function throws

If your result function `throw`s 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`.

import areInputsEqual from './are-inputs-equal';

// Using ReadonlyArray<T> rather than readonly T as it works with TS v3

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

  // Need to use 'any' rather than 'unknown' here as it has

  // The correct Generic narrowing behaviour.

  // eslint-disable-next-line @typescript-eslint/no-explicit-any

  ResultFn extends (this: any, ...newArgs: any[]) => ReturnType<ResultFn>

>(resultFn: ResultFn, isEqual: EqualityFn = areInputsEqual): ResultFn {

  // breaking cache when context (this) or arguments change

  function memoized(this: unknown, ...newArgs: unknown[]): ReturnType<ResultFn> {

    if (calledOnce && lastThis === this && isEqual(newArgs, lastArgs)) {

    // Throwing during an assignment aborts the assignment: https://codepen.io/alexreardon/pen/RYKoaz

    // Doing the lastResult assignment first so that if it throws

    lastResult = resultFn.apply(this, newArgs);

		{
		"name": "memoize-one",
		"version": "5.1.0",
		"version": "5.1.1",
		"description": "A memoization library which only remembers the latest invocation",
		@@ -5,0 +5,0 @@ "main": "dist/memoize-one.cjs.js",

		@@ -62,3 +62,3 @@ # memoize-one
		```ts
		type EqualityFn = (newArgs: readonly unknown[], lastArgs: readonly unknown[]) => boolean;
		type EqualityFn = (newArgs: any[], lastArgs: any[]) => boolean;

		@@ -65,0 +65,0 @@ // You can import this type from memoize-one if you like

		@@ -1,2 +0,2 @@
		export declare type EqualityFn = (newArgs: ReadonlyArray<unknown>, lastArgs: ReadonlyArray<unknown>) => boolean;
		export declare type EqualityFn = (newArgs: any[], lastArgs: any[]) => boolean;
		export default function memoizeOne<ResultFn extends (this: any, ...newArgs: any[]) => ReturnType<ResultFn>>(resultFn: ResultFn, isEqual?: EqualityFn): ResultFn;

		import areInputsEqual from './are-inputs-equal';

		// Using ReadonlyArray<T> rather than readonly T as it works with TS v3
		export type EqualityFn = (
		newArgs: ReadonlyArray<unknown>,
		lastArgs: ReadonlyArray<unknown>,
		) => boolean;
		export type EqualityFn = (newArgs: any[], lastArgs: any[]) => boolean;

		@@ -12,3 +9,2 @@ export default function memoizeOne<
		// The correct Generic narrowing behaviour.
		// eslint-disable-next-line @typescript-eslint/no-explicit-any
		ResultFn extends (this: any, ...newArgs: any[]) => ReturnType<ResultFn>
		@@ -15,0 +11,0 @@ >(resultFn: ResultFn, isEqual: EqualityFn = areInputsEqual): ResultFn {

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