A Node program of mine that worked fine in Node v21 now immediately crashes with a syntax error in Node v22. The line that triggers the error looks like this:
import config from "./some-config-file.json" assert { type: "json" };
and the error says:
SyntaxError: Unexpected identifier 'assert'
at compileSourceTextModule (node:internal/modules/esm/utils:337:16)
at ModuleLoader.moduleStrategy (node:internal/modules/esm/translators:166:18)
at callTranslator (node:internal/modules/esm/loader:436:14)
at ModuleLoader.moduleProvider (node:internal/modules/esm/loader:442:30)
at async ModuleJob._link (node:internal/modules/esm/module_job:106:19)
Why am I getting this error in code that previously worked, and how should I fix it?
2
This is due to support for “import assertions” being removed from Node in favour of “import attributes”, which are essentially the same thing except you have to use the keyword with instead of assert:
import config from "./some-config-file.json" with { type: "json" };
(That trivial explanation of the change might sound like it must be missing something, but at least for now and in the context of Node, it really isn’t. The change happened in Node because the underlying ECMAScript spec proposal made the same name and keyword change, on the grounds that some new import attributes – besides type: "json" – will have behaviours not properly described as “asserting” anything. But Node still supports only one one import attribute, type: "json", which behaves exactly the same as it did with the old assert syntax.)
To fix the error, you could just adopt the new syntax (i.e. change assert to with). However, do note that support for the “import attributes” syntax (with with) was added later than the older “import assertions” syntax (with assert); the latter has been around since Node 17.1 and 16.14, while the former was only added in Node 18.20. Thus, swapping from assert to with will fix your program for Node 22 and later at the expense of breaking it for Node 16 and 17.
If avoiding breaking compatibility with old Node versions matters to you, consider instead not using import for reading JSON files at all and instead reading and parsing them explicitly:
import fs from 'fs';
const config = JSON.parse(fs.readFileSync('./some-config-file.json', 'utf8'));
It’s less pretty, but more broadly compatible.