toolsharp.dev/guides/unexpected-end-of-json-input-explained

"Unexpected End of JSON Input"

This is not the same bug as "Unexpected token in JSON". That one means the parser found something it didn't expect partway through a document. This one means it ran out of document before finding anything at all — JSON.parse('') and JSON.parse('{"a":') both throw exactly this, for the same underlying reason: there's less input than a complete JSON value requires.

reference guide JavaScript fetch API

// where this actually comes from in practice

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Almost nobody hits this by calling JSON.parse() directly on an empty string on purpose — it shows up through fetch(), where response.json() calls JSON.parse() internally on whatever the body turned out to be.

the three scenarios that actually produce this
204 No Contentthe request succeeded and deliberately has no body — common on DELETE endpoints and some PUT/POST responses. Calling .json() on it parses an empty string
A network failure mid-responsethe connection dropped before the body finished arriving — the parser has a legitimate JSON prefix and then nothing, which is exactly "ran out of input"
An endpoint that just doesn't return a body on errora 500 or timeout where the server sends headers but no content — different from a server sending an HTML error page, which instead throws "Unexpected token <", not this
what actually happens, step by step
the code
const res = await fetch("/api/widgets/42", { method: "DELETE" });
const data = await res.json();  // res.json() calls JSON.parse() internally
what happens
SyntaxError: Unexpected end of JSON input
why
A successful DELETE conventionally returns 204 No Content -- no body at all. res.json() still tries to parse the (empty) body, which is the same as calling JSON.parse("").
the fix
const res = await fetch("/api/widgets/42", { method: "DELETE" });
const text = await res.text();          // read as text first, never throws
const data = text ? JSON.parse(text) : null;  // only parse if something's actually there
The general pattern for any fetch() call, not just DELETE: read the body as text once, check it's non-empty, then parse. This also means you keep the raw text to log when something does go wrong, instead of losing it the moment a failed .json() call throws.

// frequently asked questions

What does "Unexpected end of JSON input" actually mean?

The parser ran out of document before finding anything at all, unlike "Unexpected token" which means it found something it didn't expect partway through. JSON.parse('') and JSON.parse('{"a":') both throw this for the same reason: there's less input than a complete JSON value requires.

Why does res.json() throw "Unexpected end of JSON input" after a fetch() call?

Almost nobody hits this by calling JSON.parse() on an empty string directly — it shows up through fetch(), where response.json() calls JSON.parse() internally on whatever the body turned out to be. A 204 No Content response (common on DELETE endpoints), a network failure mid-response, or an endpoint that sends headers but no content on error all produce this.

How do I safely parse a fetch() response that might be empty?

Read the body as text first, check it's non-empty, then parse: const text = await res.text(); const data = text ? JSON.parse(text) : null;. This never throws on an empty body, and it also keeps the raw text around to log if something does go wrong.

// try it yourself

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Paste a response body into the JSON Formatter to see exactly what's actually in it before deciding how to handle the empty/partial case.