http.etag.formatETag

Format an ETag object to its RFC 9110 §8.8.3 HTTP header string representation.

Signature

function formatETag(tag: ETag): string

Type Definitions:

• ETag — interface

Problem

Serializing ETag objects back to HTTP header format requires correct quoting and W/ prefix handling. Ad-hoc string concatenation risks malformed headers.

How It Works

Produces the correct header format: strong ETags as "value", weak ETags as W/"value".

Boundaries

• Throws TypeError if tag is null/undefined or tag.value is not a string.

Replaces

Common boilerplate this function replaces:

tag.weak ? `W/"${tag.value}"` : `"${tag.value}"`

Examples

http.etag.formatETag({ weak: false, value: "abc" });  // '"abc"'
http.etag.formatETag({ weak: true, value: "abc" });  // 'W/"abc"'

Standards

• RFC 9110 §8.8.3 — ETag

Caveats

• Does not validate the opaque-tag value — any string is accepted.

FAQ

How to format an ETag for an HTTP response header?

http.etag.formatETag({ weak: false, value: 'abc' }) returns '"abc"'. Use this to set the ETag response header.

How to create a weak ETag header?

http.etag.formatETag({ weak: true, value: 'abc' }) returns 'W/"abc"'.

What happens with an invalid ETag object?

Throws TypeError for null, undefined, or objects without a string value property.

Is formatETag the inverse of parseETag?

Yes. formatETag(parseETag(header)) produces the original header for valid inputs.

Does formatETag validate the value content?

No. Any string is accepted as the opaque-tag value.