demo · v142

JSON Module Import Tester

Pick a MIME type variant, then attempt a dynamic import() with { with: { type: "json" } } backed by a data URL carrying that MIME. Chrome 142 tightens the token validation — trailing semicolons and invalid charset parameters now cause the import to fail.

Result

MIME used—
outcome—
strict verdict—
error—
data loaded—
Chrome 141 behaviour: —
Chrome 142 behaviour: —

MIME variant matrix

MIME type Chrome ≤141 Chrome 142+ Reason

What changed

Chrome 142 applies stricter RFC 9110 / MIME token validation when loading JSON modules. A valid MIME type for a JSON module must have:

A valid type/subtype token — no uppercase (case-folded to lower), no special characters.
Parameters separated by ; — each parameter must be a valid token=value pair with no empty values and no dangling semicolons.
Trailing semicolons (application/json;) are now rejected — they imply an empty parameter that has no name or value.
Comma-separated MIME lists (a/b, c/d) are rejected — HTTP allows multiple types in an Accept header, but a Content-Type must have exactly one type.
Unknown or non-JSON subtypes (e.g. text/plain) were already rejected; this remains the same.

The stricter validation prevents ambiguous server responses from being silently parsed as JSON, reducing supply-chain attack surface.

how the test works

// Build a data URL with the selected MIME type
const json = JSON.stringify({ test: true, ts: Date.now() });
const dataUrl = `data:${mimeType},${encodeURIComponent(json)}`;

// Dynamic import with JSON assertion
const mod = await import(dataUrl, { with: { type: "json" } });
// If MIME validation fails in Chrome 142, this throws a TypeError

see also