Description

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Advanced · 12 min

Sequence asynchronous work

Choose sequential or parallel execution based on dependencies.

Wait between dependent steps

Use await inside a for...of loop when each operation should finish before starting the next. forEach() does not wait for an async callback.

const results = [];
for (const value of [1, 2, 3]) {
  const result = await Promise.resolve(value * 2);
  results.push(result);
}
console.log(results.join(", "));

Output

2, 4, 6

Collect independent work

map() can produce an array of promises, then Promise.all() waits for all of them. Limit concurrency for large real workloads instead of starting unbounded work.

const results = await Promise.all([2, 4].map(async value => value + 1));
console.log(results.join(", "));

Output

3, 5

Put it into practice

  1. Predict each result before running the example.
  2. Complete the coding task, then explain why your solution works.

Try it yourself

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Wait for every calculation

Wait for the array of promises before summing the returned values. Display total.

Practice