sneakier_memo.ts

const _getExpensiveThing = () => {
  const c: any = getExpensiveThing;
  if (c.cache) {
    return c.cache;
  }
  c.cache = expensiveThing();
  return c.cache;
}

function memoize<T>(fn: () => T) {
  let cache: T | undefined = undefined;
  return () => {
    if (cache === undefined) {
      cache = fn();
    }
    return cache;
  };
}

interface ExpensiveThing {
  bar: number;
  baz: number;
}

function expensiveThing(): ExpensiveThing {
  console.log('computing expensive thing');
  return { bar: 1, baz: 1 };
}
const getExpensiveThing = memoize(expensiveThing);

function tt() {
  console.log(getExpensiveThing());
  console.log(getExpensiveThing());
}
tt();

test.ts

interface ExpensiveThing {
  bar: number;
  baz: number;
}

function expensiveThing(): ExpensiveThing {
  return { bar: 1, baz: 1 };
}

const getExpensiveThing = () => {
  const c: any = getExpensiveThing;
  if (c.cache) {
    return c.cache;
  }
  c.cache = expensiveThing();
  return c.cache;
}