Node.js ERR_INVALID_ARG_TYPE - Fix "argument must be of type string"

ERR_INVALID_ARG_TYPE is Node’s core APIs validating their inputs. It almost always means a value that should be a string/Buffer arrived as undefined - a missing env var, an unresolved path, or a misread config.

What this error means

A script crashes with TypeError [ERR_INVALID_ARG_TYPE], typically "The \"path\" argument must be of type string. Received undefined", in a fs/path call. It frequently fires only in CI, where an env var or config value is unset.

Node output

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type
string or an instance of Buffer or URL. Received undefined
    at fs.readFileSync (node:fs)
    at loadConfig (/app/scripts/build.js:12:18)
    code: 'ERR_INVALID_ARG_TYPE'

Common causes

A required env var is undefined in CI

Code passes process.env.SOMETHING straight into a path/fs call. The var is set locally but missing in the CI job, so undefined reaches the API.

An async/optional value used before it resolves

A path computed from a promise, argv, or config that came back empty is forwarded to a core API, which rejects the wrong type.

How to fix it

Validate inputs and surface the real cause

Fail fast with a clear message instead of letting undefined reach a core API.

JavaScript

const dir = process.env.OUTPUT_DIR
if (typeof dir !== 'string' || !dir) {
  throw new Error('OUTPUT_DIR is not set')
}
fs.readFileSync(path.join(dir, 'config.json'))

Set the missing CI value

Identify which argument is undefined from the trace, then trace it to its source.
Provide the missing env var/secret or argv in the workflow.
Add a default or guard so the failure is explicit, not a cryptic type error.

How to prevent it

Validate required env vars at startup with clear errors.
Keep CI env parity with local for required values.
Guard core-API arguments rather than passing values through blindly.