Translating curl flags into fetch() options
'curl -X POST https://api.example.com/v1/hooks -H "Content-Type: application/json" -d \'{"ok":true}\'' converts into a fetch() call with the same URL, method: "POST", the header object built from -H, and the -d payload mapped to the body option.
Each curl flag maps to a specific fetch() option: -X (or an implied POST when -d is present) becomes method, each -H becomes an entry in the headers object, and -d becomes the body -- this direct one-to-one mapping is why straightforward curl commands convert cleanly.
curl defaults to GET unless -d or -X specifies otherwise, and fetch() defaults to GET the same way -- this parity between the two tools' defaults is part of why simple commands translate predictably, without needing an explicit method flag in either direction.
Where this conversion breaks down
Simple curls only. Pipes, file uploads, and cookie jars are not parsed. curl supports many flags this converter doesn't handle: --data-binary for raw file uploads, -b/--cookie for sending a cookie jar, --form for multipart form data, and shell pipes chaining curl's output into another command -- none of which have a simple, direct fetch() equivalent.
Complex curl commands built for scripting (with shell variable substitution, multiple chained flags, or escaped quotes within escaped quotes) can confuse a straightforward flag-parsing approach, especially around quote and escape handling that differs between shell syntax and JavaScript string literals.
fetch() by default doesn't include credentials (cookies) on cross-origin requests the way a browser's native form submission or curl's cookie jar might -- if the original curl command relied on cookie-based auth, the converted fetch() call needs an explicit credentials: 'include' option added manually.
Related API testing and conversion tools
Going the opposite direction -- from a saved API collection into runnable curl commands -- the Postman Collection to curl Converter handles that.
If the header logic itself (CORS, CSP) is what's blocking the request once converted to fetch(), the CORS Header Generator addresses that separately.
Frequently Asked Questions (FAQ)
How does -X POST become method: "POST" in the output?
The converter recognizes curl's -X flag (or infers POST when -d is present without an explicit -X) and maps it directly to fetch()'s method option, since both tools use the same standard HTTP method names.
Does this handle file uploads (curl's -F or --data-binary flags)?
No. Simple curls only. Pipes, file uploads, and cookie jars are not parsed. Multipart form uploads and raw binary data flags aren't converted, since they require constructing a FormData object or handling binary data in JavaScript, which doesn't map onto a simple string-based body option.
Does this handle curl commands that send cookies (-b or --cookie)?
No. Simple curls only. Pipes, file uploads, and cookie jars are not parsed. If the original request relied on a cookie jar for authentication, the converted fetch() call needs a manually added credentials: 'include' option, which this converter doesn't add automatically.
Why might a complex curl command with lots of flags convert incorrectly?
Shell-specific syntax (variable substitution, nested quotes, escaped characters) doesn't always map cleanly onto JavaScript string literal rules -- the more flags and quoting complexity in the original command, the more likely a straightforward flag-by-flag conversion produces something needing manual correction.
Does the converted fetch() call include error handling?
No. The output is a direct translation of the curl command's request configuration -- .then()/.catch() error handling or async/await wrapping around the fetch() call needs to be added separately based on your actual application's code style.
