async/await gives Promise-based work the shape of ordinary, step-by-step JavaScript. An async function returns a Promise; inside it, await lets the next statement use an asynchronous result as a local value. The code looks as if it pauses, but Promises and the event loop still handle the work. section 8.4 section 1.5
Use it when a task has dependent steps: check access to a file, read it, transform its contents, then return a result. A Promise chain expresses the same flow with successive then calls. async/await separates those calls into statements without changing the underlying Promise-based behavior; callers can still use then on the returned Promise. section 8.4
To turn a Promise chain into an async function:
- Add async to the function declaration so callers know it returns a Promise. section 8.4
- Replace each then step with an awaited result and an explicit local variable where the next step needs that value. section 8.4
- Move catch logic into a try/catch around the asynchronous steps. section 8.4
Listing 8.21 Using async/await to count blocks in blocks.txt
const fsp = fs.promises;
async function countBlocksInFile(file) { #1
try {
await fsp.access(file, fs.constants.F_OK | fs.constants.R_OK); #2
const data = await fsp.readFile(file); #3
const decodedData = decode('utf8', data);
const blocks = tokenize(';', decodedData);
return count(blocks);
}
catch(e) { #4
throw new Error(`File ${file} does not exist or you have
no read permissions. Details: ${e.message}`);
}
}
const result = await countBlocksInFile('blocks.txt');
result; // 3
The file example makes the conversion concrete: await checks access and reads the file, ordinary functions transform its contents, and try/catch supplies an error message. section 8.4
await normally belongs inside an async function. Top-level await is the exception for ECMAScript Modules: a module can await startup work, such as a dynamic import, without wrapping it in an immediately invoked async function. A module that imports it waits for that work to finish, so consider the effect of awaiting during module initialization. section 8.6 section 8.7
