lazy-get-decorator

Lazy Get decorator

Previously known as typescript-lazy-get-decorator.

Compatibility

• Typescript - full
• Spec-compliant decorator proposal - full
• Babel (current proposal) - full
• Babel (legacy) - full

API

/**
 * Evaluate the getter function and cache the result
 * @param {boolean} [setProto=false] Set the value on the class prototype as well. Only applies to non-static getters.
 * @param {boolean} [makeNonConfigurable=false] Set to true to make the resolved property non-configurable
 * @param {ResultSelectorFn} [resultSelector] A filter function that must return true for the value to cached
 * @return A Typescript decorator function
 */
function LazyGetter(setProto?: boolean, makeNonConfigurable?: boolean, resultSelector?: (value: any) => boolean): MethodDecorator;

Usage

import {LazyGetter} from 'lazy-get-decorator';

class AClass {

    @LazyGetter()
    get lazyNoProto(): string {
        console.log('Evaluating lazyNoProto');

        return 'lazyNoProtoValue';
    }

    @LazyGetter(true)
    get lazyWithProto(): string {
        console.log('Evaluating lazyWithProto');

        return 'lazyWithProtoValue';
    }
}

const inst1 = new AClass();

console.log('==== inst 1 ====\n');

console.log(inst1.lazyNoProto);
console.log(inst1.lazyNoProto);
console.log(inst1.lazyWithProto);
console.log(inst1.lazyWithProto);

const inst2 = new AClass();

console.log('\n\n==== inst 2 ====\n');

console.log(inst2.lazyNoProto);
console.log(inst2.lazyNoProto);
console.log(inst2.lazyWithProto);
console.log(inst2.lazyWithProto);

Output:

==== inst 1 ====

Evaluating lazyNoProto
lazyNoProtoValue
lazyNoProtoValue
Evaluating lazyWithProto
lazyWithProtoValue
lazyWithProtoValue


==== inst 2 ====

Evaluating lazyNoProto
lazyNoProtoValue
lazyNoProtoValue
lazyWithProtoValue
lazyWithProtoValue

Using the result selector

import {LazyGetter} from 'lazy-get-decorator';

class MyClass {
  public readonly someCondition = 10;
  
  @LazyGetter(false, false, (v: number) => v === 10)
  public get prop1(): number {
    // This will get cached
    return this.someCondition;
  }
  
  @LazyGetter(false, false, (v: number) => v === 1)
  public get prop2(): number {
    // This won't get cached
    return this.someCondition;
  }
}

Resetting LazyGetter

The cached value can be reset if the decorator does not modify the class prototype, i.e. is not called as @LazyGetter(true):

import {LazyGetter} from 'lazy-get-decorator';

const instanceDec = LazyGetter();
const staticDec = LazyGetter();

class MyClass {
  public instanceCount = 0;
  public static staticCount = 0;
  
  @instanceDec
  public get count(): number {
    return this.instanceCount++;
  }
  
  @staticDec
  public static get count(): number {
    return MyClass.staticCount++;
  }
}

const inst = new MyClass();

console.log(inst.count); // 0
console.log(inst.count); // 0
instanceDec.reset(inst);
console.log(inst.count); // 1

console.log(MyClass.count); // 0
console.log(MyClass.count); // 0
staticDec.reset(MyClass);
console.log(MyClass.count); // 1

Resetting the decoration performs the following steps:

• Resets the property descriptor to its state before the decoration
• Re-applies the decorator

This means that any descriptor changes made by other decorators may be lost, therefore you should ensure LazyGetter is applied last if you intend on resetting it, i.e. place it at the very top of your decorators list:

class MyClass {

  @LazyGetter()
  @decorator2
  @decorator1
  get getter() {
    return 1;
  }
}