Status: Works

The fetch spec: https://developer.mozilla.org/en-US/docs/Web/API/fetch describes ten different error modes that all throw “TypeError”. Nine of these are coding errors and only one is “throws a network error”. There is no defined way of telling the difference.
This function provides a wrapper to aim to detect known coding errors in advance.


An example can be found in myfetch.tsx. This is an example module containing a React element to implement a cat facts lookup. When supplying a valid URL, the expected result is given. When a URL is invalid, the output will clearly explain why.

Error Types


  • URL Fails to parse as a new URL(), eg fetch_with_checks("\x00")
  • URL Contains a credentials, eg fetch_with_checks("https://user:[email protected]/")
  • URL Contains invalid scheme, eg fetch_with_checks("hxxp://example.com")


All variations of invalid headers will throw this error, examples from the API guide:

// space in "C ontent-Type"
const headers = {
    "C ontent-Type": "text/xml",
    "Breaking-Bad": "<3"

const headers = [
    ["Content-Type", "text/html", "extra"],


  • Provided mode is not from a valid list, eg fetch_with_checks("https://example.com/", { mode: "not a mode" })
  • Mode is no-cors but method is not a cors-safe method, eg fetch_with_checks("https://example.com/", { mode: "no-cors", method: 'CONNECT' })


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