npm package: @inventivetalent/typescript-memoize
Description: Memoize decorator for Typescript
Author: Darryl Hodgins
Latest version: 1.0.0-alpha.5

# typescript-memoize

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A memoize decorator for Typescript.

## Installation

```
npm install --save typescript-memoize
```

## Usage:

```typescript
@Memoize(hashFunction?: (...args: any[]) => any)
```

You can use it in four ways:

* Memoize a `get` accessor,
* Memoize a method which takes no parameters,
* Memoize a method which varies based on its first parameter only,
* Memoize a method which varies based on some combination of parameters

You can call memoized methods *within* the same class, too.  This could be useful if you want to memoize the return value for an entire data set, and also a filtered or mapped version of that same set.

## Memoize a `get` accessor, or a method which takes no parameters

These both work the same way. Subsequent calls to a memoized method without parameters, or to a `get` accessor, always return the same value.

I generally consider it an anti-pattern for a call to a `get` accessor to trigger an expensive operation.  Simply adding `Memoize()` to a `get` allows for seamless lazy-loading.

```typescript
import {Memoize,MemoizeExpiring} from 'typescript-memoize';

class SimpleFoo {

    // Memoize a method without parameters
    @Memoize()
    public getAllTheData() {
       // do some expensive operation to get data
       return data;
    }

    // Memoize a method and expire the value after some time
    @MemoizeExpiring(5000)
    public getDataForSomeTime() {
        // do some expensive operation to get data
        return data;
    }

    // Memoize a getter
    @Memoize()
    public get someValue() {
        // do some expensive operation to calculate value
        return value;
    }

}
```

And then we call them from somewhere else in our code:

```typescript
let simpleFoo = new SimpleFoo();

// Memoizes a calculated value and returns it:
let methodVal1 = simpleFoo.getAllTheData();

// Returns memoized value
let methodVal2 = simpleFoo.getAllTheData();

// Memoizes (lazy-loads) a calculated value and returns it:
let getterVal1 = simpleFoo.someValue;

// Returns memoized value
let getterVal2 = simpleFoo.someValue;

```

## Memoize a method which varies based on its first parameter only

Subsequent calls to this style of memoized method will always return the same value.

I'm not really sure why anyone would use this approach to memoize a method with *more* than one parameter, but it's possible.

```typescript
import {Memoize} from 'typescript-memoize';

class ComplicatedFoo {

	// Memoize a method without parameters (just like the first example)
	@Memoize()
	public getAllTheData() {
		// do some expensive operation to get data
		return data;
	}

	// Memoize a method with one parameter
	@Memoize()
	public getSomeOfTheData(id: number) {
		let allTheData = this.getAllTheData(); // if you want to!
		// do some expensive operation to get data
		return data;
	}

	// Memoize a method with multiple parameters
	// Only the first parameter will be used for memoization
	@Memoize()
	public getGreeting(name: string, planet: string) {
		return 'Hello, ' + name + '! Welcome to ' + planet;
	}

}
```

We call these methods from somewhere else in our code:

```typescript
let complicatedFoo = new ComplicatedFoo();

// Returns calculated value and memoizes it:
let oneParam1 = complicatedFoo.getSomeOfTheData();

// Returns memoized value
let oneParam2 = complicatedFoo.getSomeOfTheData();

// Memoizes a calculated value and returns it:
// 'Hello, Darryl! Welcome to Earth'
let greeterVal1 = complicatedFoo.getGreeting('Darryl', 'Earth');

// Ignores the second parameter, and returns memoized value
// 'Hello, Darryl! Welcome to Earth'
let greeterVal2 = complicatedFoo.getGreeting('Darryl', 'Mars');
```

## Memoize a method which varies based on some combination of parameters

Pass in a `hashFunction` which takes the same parameters as your target method, to memoize values based on all parameters, or some other custom logic

```typescript
import {Memoize} from 'typescript-memoize';

class MoreComplicatedFoo {

	// Memoize a method with multiple parameters
	// Memoize will remember values based on keys like: 'name;planet'
	@Memoize((name: string, planet: string) => {
		return name + ';' + string;
	})
	public getBetterGreeting(name: string, planet: string) {
		return 'Hello, ' + name + '! Welcome to ' + planet;
	}
	
	// Memoize based on some other logic
	@Memoize(() => {
		return new Date();
	})
	public memoryLeak(greeting: string) {
		return greeting + '!!!!!';
	}

}
```

We call these methods from somewhere else in our code.  By now you should be getting the idea:

```typescript
let moreComplicatedFoo = new MoreComplicatedFoo();

// 'Hello, Darryl! Welcome to Earth'
let greeterVal1 = moreComplicatedFoo.getBetterGreeting('Darryl', 'Earth');

// 'Hello, Darryl! Welcome to Mars'
let greeeterVal2 = moreComplicatedFoo.getBetterGreeting('Darryl', 'Mars');

// Fill up the computer with useless greetings:
let greeting = moreComplicatedFoo.memoryLeak('Hello');

```