Error: The variable _ is undefined when trying to use the .map() function on an array

While working on my project, I encountered a "ReferenceError: _ is not defined" when using the .map function in this code snippet:

arr.map(async (elem) => {
...
});

I couldn't find any explicit mention of "_" in my code. The error trace pointed me to:

eval
webpack-internal:///./src/components/admin/NameOfMyFile.tsx (115:51)
step
node_modules/tslib/tslib.es6.js (102:0)
Object.eval [as next]
node_modules/tslib/tslib.es6.js (83:45)
asyncGeneratorStep
node_modules/@swc/helpers/src/_async_to_generator.mjs (3:0)
_next
node_modules/@swc/helpers/src/_async_to_generator.mjs (25:0)
eval
node_modules/@swc/helpers/src/_async_to_generator.mjs (32:0)

It appears that the issue lies in trying to use something from tslib.es6.js while my tsconfig.json specifies "target" as "es5". I'm unsure if this is relevant or just a distraction.

This error occurs on the client side in my Next.js project, while it's fine on the server side. Any insights or assistance would be greatly appreciated. Thank you!

typescriptecmascript-6next.jslodashecmascript-5

Answer №1

When I encountered a similar issue, I realized it was due to nesting await statements in my code. Here's an example:

const result = await fetchData(...., {
  headers: {
    Authorization: `Bearer ${await getToken()}` 
  }
})

To resolve the problem, I moved the await getToken() outside of the function like this:

const token = await getToken()
const result = await fetchData(...., {
  headers: {
    Authorization: `Bearer ${token}` 
  }
})

This simple change fixed the issue for me. Although I thought I had used nested await statements successfully before, I haven't had the chance to investigate what went wrong this time.

Answer №2

In short, it's recommended to update Next.js

I encountered a similar issue recently. The problem started after I updated the caniuse-lite database using the command 

npx browserslist@latest --update-db
.

https://i.stack.imgur.com/y4mfF.png

(There was some usage of await within an await)

The application was not running on the latest version of Next.js, so I upgraded and the issue was resolved.

Here are some key points to note:

  • I upgraded Next.js from version 12.3.0 to 12.3.4, indicating that the solution lies in patch 1, 2, 3 or 4
  • Reverting to the previous version of "caniuse-lite" eliminated the issue
  • No relevant information was found on the Next.js GitHub page (neither in issues nor discussions)

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