// 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 = '' // 15 bytes const MAX_BUILD_ID_LENGTH = 24 // Request the first 64 bytes. The Range header is inclusive of the end value. export const DOC_PREFETCH_RANGE_HEADER_VALUE = 'bytes=0-63' function escapeBuildId(buildId: string) { // 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: string, buildId: string) { 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 + '' ) } export function doesExportedHtmlMatchBuildId( partialHtmlDocument: string, buildId: string ) { // Check whether the document starts with the expected buildId. return partialHtmlDocument.startsWith( DOCTYPE_PREFIX + '' ) }