How to implement a simple lock to prevent an expensive promise from executing multiple times.

CacheExecutingPromise.js

const expensiveOperation = () => new Promise((resolve) => {
  console.log('Executing expensive operation')
  setTimeout(resolve, 1000)
})

let expensiveOperationPromise = null
let expensiveOperationResult = null

const cacheExpensiveOperation = async () => {
  if (expensiveOperationResult !== null) {
    return expensiveOperationResult
  }

  if (expensiveOperationPromise !== null) {
    return expensiveOperationPromise
  }

  expensiveOperationPromise = expensiveOperation() // no await here
  expensiveOperationResult = await expensiveOperationPromise
  expensiveOperationPromise = null
  return expensiveOperationResult
}

// Prints "Executing expensive operation" only once.
Promise
  .all([cacheExpensiveOperation(), cacheExpensiveOperation()])
  .catch(console.error)