next
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The React Framework
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JavaScript
// In output: export mode, the build id is added to the start of the HTML
// document, directly after the doctype declaration. During a prefetch, the
// client performs a range request to get the build id, so it can check whether
// the target page belongs to the same build.
//
// The first 64 bytes of the document are requested. The exact number isn't
// too important; it must be larger than the build id + doctype + closing and
// ending comment markers, but it doesn't need to match the end of the
// comment exactly.
//
// Build ids are 21 bytes long in the default implementation, though this
// can be overridden in the Next.js config. For the purposes of this check,
// it's OK to only match the start of the id, so we'll truncate it if exceeds
// a certain length.
const DOCTYPE_PREFIX = '<!DOCTYPE html>' // 15 bytes
;
const MAX_BUILD_ID_LENGTH = 24;
function escapeBuildId(buildId) {
// If the build id is longer than the given limit, it's OK for our purposes
// to only match the beginning.
const truncated = buildId.slice(0, MAX_BUILD_ID_LENGTH);
// Replace hyphens with underscores so it doesn't break the HTML comment.
// (Unlikely, but if this did happen it would break the whole document.)
return truncated.replace(/-/g, '_');
}
export function insertBuildIdComment(originalHtml, buildId) {
if (// Skip if the build id contains a closing comment marker.
buildId.includes('-->') || // React always inserts a doctype at the start of the document. Skip if it
// isn't present. Shouldn't happen; suggests an issue elsewhere.
!originalHtml.startsWith(DOCTYPE_PREFIX)) {
// Return the original HTML unchanged. This means the document will not
// be prefetched.
// TODO: The build id comment is currently only used during prefetches, but
// if we eventually use this mechanism for regular navigations, we may need
// to error during build if we fail to insert it for some reason.
return originalHtml;
}
// The comment must be inserted after the doctype.
return originalHtml.replace(DOCTYPE_PREFIX, DOCTYPE_PREFIX + '<!--' + escapeBuildId(buildId) + '-->');
}
//# sourceMappingURL=output-export-prefetch-encoding.js.map