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); // 1Resetting 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;
}
}