scope-extensions-js

Package for using Kotlin's Scope Function Extensions on JavaScript and TypeScript.

It also supports the use of the new Optional Chaining Operator, bringing the logic of Kotlin's Null Safe Calls to the JavaScript world.

Installation

Just install the package using NPM

npm i --save scope-extensions-js

and import it directly to any file.

require("scope-extensions-js");

You can also use ES6 syntax.

import "scope-extensions-js";

For browser, reference directly to node_modules path

<script src="node_modules/scope-extensions-js/dist/index.js"></script>

or use it without installation by CDNs (unpkg/jsdelivr).

<script src="https://unpkg.com/[email protected]"></script>

<script src="https://cdn.jsdelivr.net/npm/[email protected]"></script>

Note that the type="module" tag is not needed.

Usage

Simply call any value with let, also, run or apply and it'll be passed as the argument or the context of a scope function.

const obj = { name: "Daniel", age: 30 };

obj.let(it => {
    return it.age < 0 ? it.age : 0;
}).also(it => {
    console.log(it);
}); // prints 30

This way, you can execute a block of code only if a value is neither null nor undefined.

const str: string | null = await getData();

// later
str?.also(it => {
    console.log(`Already initialized: ${it}`);
}) ?? console.log("Still not initialized");

The above code is equivalent to this

if (str != null && str != undefined)
    console.log(`Already initialized: ${str!}`);
else
    console.log("Still not initialized");

The usage of takeIf & takeUnless is a bit different. You can call any value with takeIf and it will return the caller instance if the predicate is true, or undefined if it's false (and vice versa when using takeUnless).

const account = await getCurrent();

account.takeIf(it => {
    return list.includes(it.id);
})?.also(it => {
    console.log(it);
}) ?? console.log("Not included");

Differences

We could group the 4 main extensions into 2 groups of 2 each, based on both the argument type and the return value:

• let & also receive the caller instance as a function parameter, and run & apply receive the caller instance as the function context (this).
• let & run return the function result (return) value, and also & apply return the caller instance (this).

Summed up in this table:

| | it argument | this context | |--------------------|:-----------------:|:------------------:| | Returns result | let | run | | Returns this | also | apply |

Note that let & also can be called with standard lambda/arrow functions, but because JavaScript arrow functions don't have an own this context, run & apply have to be called with standard functions.

Here is an example of each one of them:

• let

const data: Array<number> | null = await idsFromFile();

const str = data?.let(it => 
    processToString(it);
) ?? "empty";

• also

const list: Array<string> = model.getNames();

const filtered = list.also(it => 
    it.slice(0, 4);
).also(it =>
    applyFilter(filter, it);
).also(console.log);

// same as
const filtered = list.also(it => {
    it.slice(0, 4);
    applyFilter(filter, it);
    console.log(it);
});

• run

const list: Array<object> | undefined = currentAcc?.getContacts();

const lastsByName = list?.run(function() {
    this.filter();
    this.reverse();
    return this.slice(0, 3);
});

• apply

const obj = { name: "Daniel", age: 30 };

obj.apply(function() {
    this.name = "Dan";
    this.age++;
    this["country"] = "Canada";
});

License

Copyright © 2020 TheDavidDelta.
This project is MIT licensed.