doc.js

const assert = require("assert");

class Doc {
  constructor(layers) {
    this.layers = layers.reduce((accumulator, current) => {
      const { id, properties } = current;
      return {
        ...accumulator,
        [id]: properties,
      };
    }, {});

    this.actions = [];
  }

  layerById(id) {
    const properties = this.layers[id];

    return {
      id,
      properties,
    };
  }

  apply(id, property, value) {
    const properties = this.layers[id] ?? {};

    this.layers[id] = {
      ...properties,
      [property]: value,
    };

    this.actions.push([id, property, value]);
  }

  undo() {
    if (this.actions.length < 2) return;

    const [id, property, value] = this.actions[this.actions.length - 2];
    const properties = this.layers[id];

    this.layers[id] = {
      ...properties,
      [property]: value,
    };

    this.actions.pop();
  }
}

const doc = new Doc([
  { id: "a", properties: { color: "red" } },
  { id: "b", properties: { shape: "rect" } },
]);

// Initialized values should be present on the layers object on bootstrap
assert.deepEqual(doc.layerById("a"), {
  id: "a",
  properties: { color: "red" },
});

assert.deepEqual(doc.layerById("b"), {
  id: "b",
  properties: { shape: "rect" },
});

doc.apply("a", "color", "green");

// New `a` color should be `green
assert.deepEqual(doc.layerById("a"), {
  id: "a",
  properties: { color: "green" },
});

doc.apply("a", "color", "blue");
doc.apply("a", "color", "gold");
doc.apply("a", "color", "fuchsia");
doc.undo();

assert.deepEqual(doc.layerById("a"), {
  id: "a",
  properties: { color: "gold" },
});

doc.undo();

assert.deepEqual(doc.layerById("a"), {
  id: "a",
  properties: { color: "blue" },
});

doc.undo();

assert.deepEqual(doc.layerById("a"), {
  id: "a",
  properties: { color: "green" },
});

doc.apply("a", "color", "magenta");

assert.deepEqual(doc.layerById("a"), {
  id: "a",
  properties: { color: "magenta" },
});

doc.undo();

assert.deepEqual(doc.layerById("a"), {
  id: "a",
  properties: { color: "green" },
});