Lab

Lazy initialization

One center, one lookup, built on demand

The lesson asked what the second fox lookup returns if the build never threw and returned an empty index instead. Make exactly the change below, the lesson’s snippet, then run it: the second fox line answers the prediction. Restore code puts it back.

// Line 33 becomes a build that never throws:
this.index ??= this.tryBuild();

// and after build() (line 40), a failed build returns an empty index:
private tryBuild(): Map<string, number[]> {
  try {
    return this.build();
  } catch {
    return new Map();
  }
}

What to look forWhat the lazy slot holds after the first build

A lazy slot keeps whatever its first build hands it. A build that catches its own failure hands it an empty index, which ??= keeps like any other value: every later lookup, after the export is back too, finds no sheet, with one build and no error. The program as written lets the failure throw, so ??= assigns nothing and the next lookup builds again: fox finds sheet 1 on the second build.

type Sheet = { id: number; species: string };

// The night's care-sheet export: still syncing when the desk opens.
let exportReady = false;

// The recipe: each word of a species name → the ids of its sheets.
function buildIndex(sheets: Sheet[]): Map<string, number[]> {
  if (!exportReady) throw new Error('export still syncing');
  const index = new Map<string, number[]>();
  for (const sheet of sheets) {
    for (const word of sheet.species.toLowerCase().split(' ')) {
      index.set(word, [...(index.get(word) ?? []), sheet.id]);
    }
  }
  return index;
}

// One center's care sheets. The lookup waits for the first lookup.
class CareSheets {
  builds = 0;
  private sheets: Sheet[];
  private index: Map<string, number[]> | undefined;

  constructor(sheets: Sheet[]) {
    this.sheets = sheets;
  }

  browse(): Sheet[] {
    return this.sheets;
  }

  lookup(word: string): number[] {
    this.index ??= this.build();
    return this.index.get(word) ?? [];
  }

  private build(): Map<string, number[]> {
    this.builds++;
    return buildIndex(this.sheets);
  }
}

const center = new CareSheets([
  { id: 1, species: 'Red fox' },
  { id: 2, species: 'River otter' },
  { id: 3, species: 'Barn owl' },
  { id: 4, species: 'Hedgehog' },
]);

function attempt(word: string): string {
  try {
    return `${word}: [${center.lookup(word).join(', ')}]`;
  } catch (error) {
    return `${word}: ${(error as Error).message}`;
  }
}

console.log(`opened: builds=${center.builds}`);
const browsed = center.browse();
console.log(`browse: ${browsed.length} sheets, builds=${center.builds}`);
const first = attempt('fox');
console.log(`${first}, builds=${center.builds}`);
exportReady = true; // the export finishes syncing
const second = attempt('fox');
console.log(`${second}, builds=${center.builds}`);
const third = attempt('owl');
console.log(`${third}, builds=${center.builds}`);

TODO

Visual to come