Typescript useful features

ts_useful_features.ts

/** 
  NoInfer type 
*/
function createStreetLight<C extends string>(
  colors: C[],
  defaultColor?: NoInfer<C>,
) {
  // ...
}
createStreetLight(["red", "yellow", "green"], "red");  // OK
createStreetLight(["red", "yellow", "green"], "blue");  // Error

/* ************************************************** */
  
/** 
  as const 

  Works with no types referenced or declared.
  We only needed a single const assertion.
*/
function getShapes() {
  let result = [
    { kind: "circle", radius: 100 },
    { kind: "square", sideLength: 50 },
  ] as const;
  return result;
}
for (const shape of getShapes()) {
  // Narrows perfectly!
  if (shape.kind === "circle") {
    console.log("Circle radius", shape.radius);
  } else {
    console.log("Square side length", shape.sideLength);
  }
}
  
  
type T0 = NonNullable<string | number | undefined>; // string | number
type T0 = ReturnType<() => string>; // string
type T0 = Extract<"a" | "b" | "c", "a" | "f">; // a

/* ************************************************** */

/** 
  Not-null assertion 

  Compiled with --strictNullChecks
*/
function validateEntity(e?: Entity) {
  // Throw exception if e is null or invalid entity
}
function processEntity(e?: Entity) {
  validateEntity(e);
  let s = e!.name; // Assert that e is non-null and access name
}

/* ************************************************** */

/** 
  Object.groupBy and Map.groupBy 
*/
const array = [0, 1, 2, 3, 4, 5];
const myObj = Object.groupBy(array, (num, index) => {
    return num % 2 === 0 ? "even": "odd";
});
// equivalent to 
const myObj = {
    even: [0, 2, 4],
    odd: [1, 3, 5],
};

const myObj = Map.groupBy(array, (num, index) => {
    return num % 2 === 0 ? "even" : "odd";
});
// equivalent to 
const myObj = new Map();
myObj.set("even", [0, 2, 4]);
myObj.set("odd", [1, 3, 5]);

/* ************************************************** */

/** 
  AsyncDisposable and using 

  Sometimes we have a need to do clean-up after our unit completed its job, this can be: deleting of temp files, 
closing network connections, free up some memory. 

  New keyword `using` applied to a variable gets its Symbol.dispose or Symbol.asyncDispose method 
to be called at the end of the scope. 
*/

async function doWork() {
    // Do fake work for half a second.
    await new Promise(resolve => setTimeout(resolve, 500));
}
function loggy(id: string): AsyncDisposable {
    console.log(`Constructing ${id}`);
    return {
        async [Symbol.asyncDispose]() {
            console.log(`Disposing (async) ${id}`);
            await doWork();
        },
    }
}
async function func() {
    await using a = loggy("a");
    await using b = loggy("b");
    {
        await using c = loggy("c");
        await using d = loggy("d");
    }
    await using e = loggy("e");
    return;
}
func();
// Constructing a
// Constructing b
// Constructing c
// Constructing d
// Disposing (async) d
// Disposing (async) c
// Constructing e
// Disposing (async) e
// Disposing (async) b
// Disposing (async) a

/** 
  DisposableStack and using 

  useful for doing both one-off clean-up, along with arbitrary amounts of cleanup.

  A DisposableStack is a Disposable object that has several methods for keeping track of Disposable objects, 
and can be given functions for doing arbitrary clean-up work.

  Method defer has a callback that will be called once cleanup is disposed.
  Once a resource is created is should be disposed. 
*/
function doSomeWork() {
    const path = ".some_temp_file";
    const file = fs.openSync(path, "w+");
    using cleanup = new DisposableStack();
    cleanup.defer(() => {
        fs.closeSync(file);
        fs.unlinkSync(path);
    });
    // use file...
    if (someCondition()) {
        // do some more work...
        return;
    }
    // ...
}

/**
  Decorator metadata:
  Exampe of this is here: 
  https://gist.github.com/Mefistophell/07d63b44d69d38af9406bc59a35c3ec7
*/