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Showing content from http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function below:

async function expression - JavaScript

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The async function keywords can be used to define an async function inside an expression.

You can also define async functions using the async function declaration or the arrow syntax.

async function (param0) {
  statements
}
async function (param0, param1) {
  statements
}
async function (param0, param1, /* …, */ paramN) {
  statements
}

async function name(param0) {
  statements
}
async function name(param0, param1) {
  statements
}
async function name(param0, param1, /* …, */ paramN) {
  statements
}

Note: An expression statement cannot begin with the keywords async function to avoid ambiguity with an async function declaration. The async function keywords only begin an expression when they appear in a context that cannot accept statements.

name Optional

The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.

paramN Optional

The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.

statements Optional

The statements which comprise the body of the function.

An async function expression is very similar to, and has almost the same syntax as, an async function declaration. The main difference between an async function expression and an async function declaration is the function name, which can be omitted in async function expressions to create anonymous functions. An async function expression can be used as an IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined, allowing you to mimic top-level await. See also the chapter about functions for more information.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

// async function expression assigned to a variable
const add = async function (x) {
  const a = await resolveAfter2Seconds(20);
  const b = await resolveAfter2Seconds(30);
  return x + a + b;
};

add(10).then((v) => {
  console.log(v); // prints 60 after 4 seconds.
});

// async function expression used as an IIFE
(async function (x) {
  const p1 = resolveAfter2Seconds(20);
  const p2 = resolveAfter2Seconds(30);
  return x + (await p1) + (await p2);
})(10).then((v) => {
  console.log(v); // prints 60 after 2 seconds.
});

An async IIFE allows you to use await and for...await in contexts where top-level await is not available. Here we use an arrow function to define the IIFE, but async function expressions can also be used.

const getFileStream = async (url) => {
  // implementation
};

(async () => {
  const stream = await getFileStream("https://domain.name/path/file.ext");
  for await (const chunk of stream) {
    console.log({ chunk });
  }
})();

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