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async and await

What are async and await?

  • async and await are syntactic sugar for working with Promises.
  • async function is syntactically closer to return new Promise(...) statement.
  • await operator is analogous to Promise.then method.
  • Together they help write asynchronous code without nested callbacks or then blocks.

async

  • async keyword/modifier wraps a function in a Promise.
  • It returns a Promise, instead of the expression in the function's return statement.
  • This promise may resolve with the value of the expression in the return statement.
  • Or it may reject with the reason of error encountered during the function execution.

await

  • await operator unwraps a Promise.
  • It extracts the resolved value if the promise is fulfilled.
  • Or it throws with rejected reason if the promise is rejected.

Usage Notes

  • An async function always returns a promise and await can access its return value.
  • async functions run synchronously till an await operation is encountered.
  • await operation defers execution of async functions, letting pending promises get settled.
  • So an async function with no await statements ends synchronously.
  • And an async function with await statements ends asynchronously.
  • await can only be used inside an async function, at least for now.

Awaiting multiple Promises or Async functions

  • async and await offer no alternative to working with Promises for handling multiple async functions or promises.
  • Promise methods all, allSettled, any and race are required to await multiple async functions or promises.

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