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How to change instance properties through decorators in TypeScript: http://romkevandermeulen.nl/2018/01/24/typescript-property-decorators.html
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function makePropertyMapper<T>(prototype: any, key: string, mapper: (value: any) => T) { | |
const values = new Map<any, T>(); | |
Object.defineProperty(prototype, key, { | |
set(firstValue: any) { | |
Object.defineProperty(this, key, { | |
get() { | |
return values.get(this); | |
}, | |
set(value: any) { | |
values.set(this, mapper(value)); | |
}, | |
enumerable: true, | |
}); | |
this[key] = firstValue; | |
}, | |
enumerable: true, | |
configurable: true, | |
}); | |
} | |
function exampleDecorator(multiplier: number) { | |
return function(target: any, key: string) { | |
makePropertyMapper(target, key, (value: number) => { | |
return value * multiplier; | |
}); | |
}; | |
} | |
class Example { | |
@exampleDecorator(3) | |
myNumber: number; | |
@exampleDecorator(3) | |
withInitializer = 2; | |
} | |
const example = new Example(); | |
console.log(example.myNumber); // undefined | |
console.log(Object.keys(example).includes("myNumber")); // false | |
console.log(example.withInitializer); // 6 | |
console.log(Object.keys(example).includes("withInitializer")); // true | |
example.myNumber = 3; | |
console.log(example.myNumber); // 9 | |
console.log(Object.keys(example).includes("myNumber")); // true |
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