@trademe/ensure

@trademe/ensure - utility decorators for Angular applications.

@Singleton:

This decorator can be applied to @Injectable classes to ensure that they are instantiated only once. This is useful for stateful services that belong at the root of the application.

Example:

import { Singleton } from '@trademe/ensure';

@Singleton
@Injectable
export class MyStatefulService { }

let instance = new MyStatefulService(); // works:
let another = new MyStatefulService(); // throws!

---

@Value:

This decorator can be applied to properties on a class to ensure that they conform to rules when they are set. The rules also act as a mechanism for casting values from HTML attributes, which makes for nicer component APIs.

Example:

import { Value } from '@trademe/ensure';

export class MyClass {
  @Value(isOne) mustBeOne: number;
  @Value([isString, isLengthFive]) verySpecificString: string;
}

export function isOne (value: any) {
    if (value === 1) {
        return value;
    }
    throw new Error();
}

export function isString (value: string) {
    if (typeof value === 'string') {
        return value;
    }
    throw new Error();
}

export function isLengthFive (value: string) {
    if (value.length === 5) {
        return value;
    }
    throw new Error();
}

Built in rules:

isNotNull

Ensure that a value is not null. isNotNull is special in that it acts when getting the value, not setting it.

Example:

import { isNotNull } from '@trademe/ensure';

export class MyClass {
    @Value(isNotNull) mustNotBeNull: boolean;
}

let instance = new MyClass();
instance.mustNotBeNull = true; // works
instance.mustNotBeNull = null; // works
console.log(instance.mustNotBeNull): // throws!

isBool

Ensure that a value is boolean. Also casts from 'true' or '' to true and 'false' to false.

Example:

import { isBool } from '@trademe/ensure';

export class MyClass {
    @Value(isBool) mustBeBoolean: boolean;
}

let instance = new MyClass();
instance.mustBeBoolean = true; // works
instance.mustBeBoolean = 'hello'; // throws!
instance.mustBeBoolean = 'false'; // works
console.log(instance.mustBeBoolean) // false

isBool is particularly useful for simplifying component APIs when combined with @Input.

Example:

import { isBool } from '@trademe/ensure';

@Component({ ... })
export class MyComponent {
    @Input() @Value(isBool) mustBeBoolean: boolean;
}

<my-component
  mustBeBoolean><!-- No more pesky ="true"! -->
</my-component>

isEnum

Ensure that a value is a valid enum value.

Example:

import { isEnum } from '@trademe/ensure';

export enum MyEnum {
    foo,
    bar
}

export class MyClass {
    @Value(isEnum<typeof MyEnum>(MyEnum)) mustBeEnum: MyEnum;
}

let instance = new MyClass();
instance.mustBeEnum = 'foo'; // works
console.log(instance.mustBeEnum); // 0
instance.mustBeEnum = 'bar'; // works!
console.log(instance.mustBeEnum); // 1
instance.mustBeEnum = 'baz'; // throws!

isNumber

Ensure that a value is a valid number.

Example:

import { isNumber } from '@trademe/ensure';

export class MyClass {
    @Value(isNumber) mustBeNumber: number;
}

let instance = new MyClass();
instance.mustBeNumber = 5; // works
console.log(instance.mustBeNumber); // 5
instance.mustBeNumber = '1.33'; // works!
console.log(instance.mustBeNumber); // 1.33
instance.mustBeNumber = 'baz'; // throws!