Late initialization decorator for TypeScript. Comes with @lateinit and @readonlyLateinit variants for controlling the mutability of the property.

Installation

TypedJSON is available from npm:

npm install late-init-js

How to Use

TypeScript needs to run with the experimentalDecorators option enabled.

You must use the definite assignment assertion if the property type is not optional (i.e not T | undefined):

class Person {
  @lateinit()
  name!: string;
  //  ^ definite assignment assertion
    
  @lateinit()
  nullableName: string | undefined;
  //  no definite assignment required
}

Mutable @lateinit Property

import { lateinit } from "late-init-js"

class Person {
  @lateinit()
  name!: string;
}

const person = new Person();
person.name = "Alice";
console.log(person.name); // prints "Alice"
person.name = "Bob";
console.log(person.name); // prints "Bob"

const anotherPerson = new Person();
console.log(anotherPerson.name); // throws a LateinitNotInitializedException

Immutable @readonlyLateinit Property

import { readonlyLateinit } from "late-init-js"

class Person {
    @readonlyLateinit()
    name!: string;
}

const person = new Person();
person.name = "Alice";
console.log(person.name); // prints "Alice"
person.name = "Bob"; // throws a ReadonlyLateinitAlreadyInitializedException

Ignoring initial set to undefined

@lateinit and @readonlyLateInit accept an optional parameter options for which if ignoreInitialUndefined is true, late-init-js will ignore the first "set" action on the property if it is undefined.

This can be helpful when using reflection to instantiate a class, for example when using serialization/deserialization libraries such as TypedJson.

import { readonlyLateinit } from "late-init-js"

class Person {
    @readonlyLateinit({ ignoreInitialUndefined: true })
    name!: string;
}

const person = new Person();
person.name = "Alice";
console.log(person.name); // prints "Alice"
person.name = "Bob"; // throws a ReadonlyLateinitAlreadyInitializedException

Checking if a Property Has Been Initialized

To check whether the property has been initialized, use the isInitialized(thisRef: any, propertyKey: string) function.

import { readonlyLateinit, isInitialized } from "late-init-js"

class Person {
    @readonlyLateinit()
    name!: string;
}

const person = new Person();
console.log(isInitialized(person, "name")); // prints "false"
person.name = "Alice";
console.log(isInitialized(person, "name")); // prints "true"

However, if you find you are using isInitialized a lot, you may wish to consider using a optional type instead to take advantage of TypeScripts optional chaining features such as ? and ??.

Notes

late-init-js adds two extra properties for each property that is defined as lateinit, prefixed with __late-init-js_INI__ and __late-init-js_VAL__, to keep track of whether the property was initialized and the value respectively.

In addition, if the ignoreInitialUndefined parameter option is enabled, an additional property prefixed with __late-init-js_SET__ is used to keep track of whether the property has been first set with undefined yet.

For example, for the above example with the property name, the two extra properties would be __late-init-js_INI__name and __late-init-js_VAL__name, and with ignoreInitialUndefined enabled, the additional property would be late-init-js_SET_name

Ensure that these properties are not overwritten.

License

late-init-js is licensed under the ISC License.