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Determine the Encoding of a HTML Byte Stream

This package implements the HTML Standard's encoding sniffing algorithm in all its glory. The most interesting part of this is how it pre-scans the first 1024 bytes in order to search for certain <meta charset>-related patterns.

const htmlEncodingSniffer = require("html-encoding-sniffer");
const fs = require("fs");

const htmlBytes = fs.readFileSync("./html-page.html");
const sniffedEncoding = htmlEncodingSniffer(htmlBytes);

The passed bytes are given as a Uint8Array; the Node.js Buffer subclass of Uint8Array will also work, as shown above.

The returned value will be a canonical encoding name (not a label). You might then combine this with the whatwg-encoding package to decode the result:

const whatwgEncoding = require("whatwg-encoding");
const htmlString = whatwgEncoding.decode(htmlBytes, sniffedEncoding);


You can pass two potential options to htmlEncodingSniffer:

const sniffedEncoding = htmlEncodingSniffer(htmlBytes, {

These represent two possible inputs into the encoding sniffing algorithm:

  • transportLayerEncodingLabel is an encoding label that is obtained from the "transport layer" (probably a HTTP Content-Type header), which overrides everything but a BOM.
  • defaultEncoding is the ultimate fallback encoding used if no valid encoding is supplied by the transport layer, and no encoding is sniffed from the bytes. It defaults to "windows-1252", as recommended by the algorithm's table of suggested defaults for "All other locales" (including the en locale).


This package was originally based on the excellent work of @nicolashenry, in jsdom. It has since been pulled out into this separate package.