Asynchronous Iteration with Javascript Async Generators
Jul 06, 2025 am 02:07 AMAsync Generator is a generator function that supports asynchronous operations. It can generate data while waiting asynchronously, and is suitable for streaming data processing. 1. It is defined by async function*, and can be directly followed by Promise after yield without manual .then(); 2. Use for await...of to consume the data stream it generates, making the code look synchronous but actually execute asynchronously; 3. Unlike ordinary asynchronous functions, it supports the gradual generation and processing of data, saving memory resources; 4. Pay attention to environmental compatibility when using it, and use for await...of to traverse, and exception handling should be cautious, and avoid interrupting the process at will. Async Generator combines the generator's process control and async/await's asynchronous capability, and is suitable for segmented acquisition and streaming scenarios.
Asynchronous iteration is especially useful when processing streaming data or getting results step by step, and JavaScript's async generators provide a more natural way to implement it. If you need to generate data while obtaining it asynchronously (such as pulling from API paging, reading large file chunking, etc.), async generator is a very suitable tool.

What is Async Generator?
async generator is a generator function that can return a Promise and support await
. Its syntax is similar to that of ordinary generators, except that the async
keyword is added before it. The value it returns every time it calls next()
is a Promise, not a direct { value, done }
object.

You can define an async generator like this:
async function* myAsyncGenerator() { yield await someAsyncOperation(); }
What's different from ordinary generators is that yield can be followed by Promise, and you don't need to go to .then()
manually.

How to do asynchronous iteration using Async Generator?
The most common way is to combine for await...of
to consume the data stream generated by the async generator. For example, if you want to get the user list by paging, each page will be obtained through fetch:
async function* fetchUserPages() { let page = 1; while (true) { const res = await fetch(`https://api.example.com/users?page=${page}`); const data = await res.json(); if (data.users.length === 0) break; yield data.users; page ; } } //Usage method for await (const users of fetchUserPages()) { console.log(users); }
The benefits of this approach are:
- The code looks synchronous, but behind it is asynchronous execution.
- Data comes from pieces one by one, suitable for big data processing.
- The loop can be interrupted at any time, such as exiting when empty data is encountered.
What is the difference between Async Generator and ordinary asynchronous functions?
Ordinary asynchronous functions return all data at once, while async generator can "wait asynchronously while generating". This is very useful when dealing with large amounts of data or streaming data. For example, you have a large log file that you want to read and analyze by line:
- Ordinary async functions may have to read the complete file at once before processing, which consumes high memory.
- async generator can read line by line, process while reading, saving resources.
In addition, async generator supports pause and recovery, controls processes like generators, and also has asynchronous capabilities.
What should you pay attention to when using it?
Although async generator is powerful, there are several details that are easy to ignore during use:
- Not all environments are fully supported : Node.js and modern browsers are mostly supported, but if you are still maintaining old projects, you have to confirm whether the running environment is compatible.
- You can't just use normal for...of traversal : you must use
for await...of
, otherwise you will get a bunch of promises instead of the actual data. - Be careful when handling exceptions : errors thrown during asynchronous generation will not be automatically caught. It is recommended to add try/catch inside the generator, or catch errors when traversing externally.
- Don't return easily, break to consider the status : If you jump out of
for await...of
, the generator may not have completed the entire process yet, so be careful whether it affects the subsequent logic.
Basically that's it. Async generator combines the generator's process control capabilities and the asynchronous advantages of async/await, and is very suitable for handling scenarios such as segmentation acquisition and streaming. As long as you pay attention to usage and compatibility, you can write clear and efficient asynchronous iterative code.
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