A Deep Dive into JavaScript async/await

async/await is syntactic sugar over Promises, but understanding what’s actually happening underneath prevents a category of subtle async bugs.

// An async function ALWAYS returns a Promise, even if you return a plain value
async function getValue() { return 42; }
getValue().then(v => console.log(v)); // 42, wrapped in a Promise automatically

// Sequential vs parallel await - a very common performance mistake
async function sequentialSlow() {
    const a = await fetchA(); // waits for A to finish...
    const b = await fetchB(); // ...THEN starts B - unnecessarily serial
    return [a, b];
}

async function parallelFast() {
    const [a, b] = await Promise.all([fetchA(), fetchB()]); // both start immediately
    return [a, b];
}

Error Handling Across Multiple Awaits

async function process() {
    try {
        const data = await fetchData();
        const parsed = await parseData(data);
        return parsed;
    } catch (err) {
        // catches a rejection from EITHER await above
        console.error('Processing failed:', err);
        throw err;
    }
}

The sequential-vs-parallel mistake above is genuinely common and easy to miss in code review — two independent await calls that don’t depend on each other should almost always be wrapped in Promise.all rather than awaited one after another.