It should work without any changes, because both module and axios itself supports Promises

https://github.com/nuxt-community/axios-module/blob/main/types/index.d.ts#L20-L21
https://github.com/axios/axios/blob/master/index.d.ts#L125-L128

Do you mind creating a reproduction sample using template.nuxtjs.org

Thank you for the response.

I'm getting an error when using async when extending onRequest in plugins.

This is the sample. Contains errors ver.
https://codesandbox.io/s/funny-meadow-zwo93?file=/plugins/axios-extension.js

This sample is trying to get the README.me in the static directory.

~/plugins/axios-extension.js

export default ({ $axios }) => {
  $axios.onRequest(async () => {
    console.log("onRequest()");
  });
};

~/pages/index.vue

// at methods
async clicked() {
  try {
    this.messages.push("Clicked");
    const { data } = await this.$axios.get("/README.md");
    this.messages.push("Finish. " + data);
  } catch (err) {
    this.messages.push("Error occurred. " + err.message);
    console.error(err);
  }
}

NOT ASYNC (Success) onRequest(()=>{})

ASYNC (Failed) onRequest(async ()=>{})

I came across the same error message when using an async function in onRequest. I think the problem might be that async functions by default return a promise (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function#return_value) but here you need to return the config object to pass it further to Axios.

Try something like this:

$axios.onRequest(async function (config) {
    try {
            await [SOMETHING]
            console.log('Awaited function finished')
        } catch (error) {
            console.log(error)
        }
    return config
  })

It took me a while to realise because the docs say "these functions don't have to return anything by default" (https://axios.nuxtjs.org/helpers) but I guess the implicitly returned promise overrides the config object which would otherwise be passed on in the background by default.

Thank you for your reply.

The code as instructed worked fine.

Just in case, I shared the sample code that I forked and modified.
https://codesandbox.io/s/priceless-cartwright-5sgj7?file=/plugins/axios-extension.js

Thank you so much!

I came across the same error message when using an async function in onRequest. I think the problem might be that async functions by default return a promise (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function#return_value) but here you need to return the config object to pass it further to Axios.

Try something like this:

$axios.onRequest(async function (config) {
    try {
            await [SOMETHING]
            console.log('Awaited function finished')
        } catch (error) {
            console.log(error)
        }
    return config
  })

It took me a while to realise because the docs say "these functions don't have to return anything by default" (https://axios.nuxtjs.org/helpers) but I guess the implicitly returned promise overrides the config object which would otherwise be passed on in the background by default.

You Sir, are a legend! That should definitely be in the docs.

I created a PR (#548) to resolve this gap in the docs