Boundary

A boundary represents everything below a given point in the call stack.

It can have an onEnter function which is called on entry and an onExit function which is called on exit.

To enter the boundry call enter with the function you want to run inside it. On the first call to enter in the current stack onEnter will be called before the provided function and onExit will be called after it. You can return a value from onEnter which will be passed to all the enter and onExit calls. This may be undefined if onEnter throws an exception or if onEnter is still executing.

See https://github.com/tjenkinson/state-manager for a use case of this.

Installation

npm install --save @tjenkinson/boundary

or available on JSDelivr at "https://cdn.jsdelivr.net/npm/@tjenkinson/boundary@2".

Example

import { Boundary } from '@tjenkinson/boundary';

class Something {
  private _boundary = new Boundary({
    onEnter: () => {
      console.log('Entering...');
      return Math.random();
    },
    onExit: ({ onEnterResult }) => {
      // we know `onEnterResult` cannot be `undefined` given `onEnter` won't throw
      console.log(`Done! Random number is ${onEnterResult!}`);
    },
  });

  public doSomething(): number {
    return this._boundary.enter((randomNumber) => {
      // we know `randomNumber` cannot be `undefined` given `onEnter` doesn't
      // call `doSomething()`.
      console.log(`Something. Random number is ${randomNumber!}`);
      return randomNumber * 2;
    });
  }

  public doSomethingAndSomethingElse(): number {
    return this._boundary.enter((randomNumber) => {
      const randomTimesTwo = this._doSomething();
      console.log(`Something else. Random number is ${randomNumber!}`);
      return randomTimesTwo + 1;
    });
  }
}

const something = new Something();

something.doSomething();
// -> Entering...
// -> Something. Random number is 0.2
// -> Done! Random number is 0.2
// Return value: 0.4

something.doSomethingAndSomethingElse();
// -> Entering...
// -> Something. Random number is 0.6
// -> Something else. Random number is 0.6
// -> Done! Random number is 0.6
// Return value: 2.2