Top-level await

Top-level await in ECMAScript 13

ECMAScript 13 introduces top-level await, allowing the direct use of the await keyword at the top level of a module without wrapping it in an async function. This feature simplifies the writing of asynchronous code, particularly when asynchronous operations need to be completed during the module initialization phase.

Background of Top-level await

In ES2017, await could only be used inside async functions. This meant that if asynchronous operations needed to be awaited at the top level of a module, the code had to be wrapped in an async function and immediately invoked. For example:

// ES2017 approach
(async () => {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  console.log(data);
})();

While this approach worked, it added unnecessary nesting and complexity. ECMAScript 13's top-level await solves this problem.

Basic Usage of Top-level await

In environments that support top-level await, you can use await directly at the top level of a module:

// ECMAScript 13 approach
const response = await fetch('https://api.example.com/data');
const data = await response.json();
console.log(data);

This syntax is more concise and intuitive, especially suitable for scenarios where asynchronous resource loading is required during module initialization.

Execution Order with Top-level await

Top-level await affects the execution order of modules. When a module contains top-level await, code importing that module will wait for the top-level await to complete before continuing execution. For example:

// moduleA.js
console.log('Module A starts');
await new Promise(resolve => setTimeout(resolve, 1000));
console.log('Module A after await');

// moduleB.js
import './moduleA.js';
console.log('Module B');

The execution order will be:

1. "Module A starts"
2. (Wait for 1 second)
3. "Module A after await"
4. "Module B"

Practical Use Cases for Top-level await

Dynamic Module Imports

Top-level await is particularly useful when combined with dynamic imports:

const module = await import('./module.js');
module.doSomething();

Configuration Loading

Loading configurations during application startup:

const config = await fetch('/config.json').then(res => res.json());
export default config;

Database Connections

Initializing database connections in server-side JavaScript:

const connection = await connectToDatabase();
export const db = connection;

Error Handling

Error handling for top-level await can be implemented using try/catch:

try {
  const data = await fetchData();
  console.log(data);
} catch (error) {
  console.error('Failed to fetch data:', error);
}

Browser Compatibility

Most modern browsers now support top-level await. In unsupported environments, transpilation tools like Webpack or Rollup are required.

Performance Considerations

When using top-level await, keep the following in mind:

1. It blocks module evaluation until the await expression completes.
2. Overuse may cause delays in application startup.
3. Avoid long-running top-level await operations.

Differences from CommonJS

Node.js's CommonJS module system does not support top-level await. To use top-level await in Node.js:

1. Use ES modules (with the .mjs file extension or by setting "type": "module" in package.json).
2. Node.js version 14.8.0 or higher is required.

Module Dependency Relationships

Top-level await affects module dependency resolution order. Consider the following example:

// a.js
export const value = await Promise.resolve(42);

// b.js
import { value } from './a.js';
console.log(value); // 42

Module b.js will wait for the top-level await in a.js to complete before executing.

Behavior in Circular Dependencies

Special attention is needed for top-level await behavior in circular dependencies:

// a.js
import { b } from './b.js';
export const a = 'a' + b;

// b.js
import { a } from './a.js';
await new Promise(resolve => setTimeout(resolve, 100));
export const b = 'b' + a;

This scenario may lead to deadlocks, so avoid using top-level await in circular dependencies whenever possible.

Usage in Testing

Top-level await can simplify the writing of asynchronous tests:

// test.js
const result = await someAsyncFunction();
assert.equal(result, expectedValue);

Comparison with Static Imports

Static import statements are processed before any code execution, while top-level await allows dynamic determination of imported content:

const moduleName = await determineModuleName();
const module = await import(moduleName);

Limitations and Considerations

1. Top-level await can only be used in modules, not in scripts.
2. It cannot be used in non-module contexts (e.g., <script> tags without type="module").
3. It affects module loading timing and order.
4. It may obscure performance issues, as waiting times are not immediately obvious.

Interaction with Other Language Features

Top-level await can be used with most ES features:

// Combined with class static blocks
class MyClass {
  static {
    const data = await fetchData();
    this.data = data;
  }
}

Debugging Tips

When debugging top-level await:

1. Use debugger statements to pause execution.
2. Error stacks will include the location of the top-level await.
3. Use console.time to measure the duration of await operations.

Build Tool Support

Major build tools' support for top-level await:

1. Webpack 5+ supports it natively.
2. Rollup requires plugins.
3. Babel can transpile it via presets.
4. TypeScript 3.8+ supports it.

Applications in Server-Side Rendering

Using top-level await in SSR frameworks to fetch data:

// Page component
const data = await fetchServerData();
export default function Page() {
  return `<div>${data}</div>`;
}

Integration with Web Workers

Using top-level await for initialization in Web Workers:

// worker.js
const heavyData = await processHeavyData();
self.onmessage = (e) => {
  // Process messages using heavyData
};