webpack/lib/util/removeBOM.js

Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.

/*
	MIT License http://www.opensource.org/licenses/mit-license.php
	Author Alexander Akait @alexander-akait
*/

"use strict";

const { decodeVLQ, encodeVLQ } = require("./createMappings");

/** @import { RawSourceMap } from "webpack-sources" */
/** @import { Result } from "../NormalModule" */

/**
 * Returns result without BOM.
 * @param {string | Buffer} strOrBuffer string or buffer
 * @returns {string | Buffer} result without BOM
 */
const removeBOM = (strOrBuffer) => {
	if (typeof strOrBuffer === "string" && strOrBuffer.charCodeAt(0) === 0xfeff) {
		return strOrBuffer.slice(1);
	} else if (
		Buffer.isBuffer(strOrBuffer) &&
		strOrBuffer[0] === 0xef &&
		strOrBuffer[1] === 0xbb &&
		strOrBuffer[2] === 0xbf
	) {
		return strOrBuffer.subarray(3);
	}

	return strOrBuffer;
};

/**
 * A BOM occupies generated column 0 of the first line, so a map describing
 * BOM-prefixed content starts that line at column 1 or later. Removing the BOM
 * moves the line one column left; the segments after the first are encoded
 * relative to it, so only the first is rewritten. A map already starting at
 * column 0 describes the content without its BOM and is left alone.
 * @param {string} mappings VLQ encoded `mappings` of a map for BOM-prefixed content
 * @returns {string} `mappings` for the same content without its BOM
 */
const shiftMappings = (mappings) => {
	const { value, end } = decodeVLQ(mappings, 0);
	if (end === 0 || value <= 0) return mappings;
	return encodeVLQ(value - 1) + mappings.slice(end);
};

/**
 * Adjust a source map that accompanied BOM-prefixed content for the removal of
 * that BOM. Anything without VLQ `mappings` is passed through untouched, as is
 * a map declaring a version other than 3 — earlier versions spell `mappings`
 * differently. A map that declares no version at all is taken as version 3.
 * @param {(string | RawSourceMap)=} sourceMap the map, as an object or as JSON
 * @returns {(string | RawSourceMap)=} the adjusted map
 */
const adjustSourceMapForRemovedBOM = (sourceMap) => {
	if (!sourceMap) return sourceMap;

	if (typeof sourceMap === "string") {
		/** @type {RawSourceMap} */
		let parsed;
		try {
			parsed = JSON.parse(sourceMap);
		} catch (_err) {
			return sourceMap;
		}
		const adjusted = adjustSourceMapForRemovedBOM(parsed);
		return adjusted === parsed ? sourceMap : JSON.stringify(adjusted);
	}

	if (sourceMap.version !== undefined && sourceMap.version !== 3) {
		return sourceMap;
	}

	if (typeof sourceMap.mappings !== "string") return sourceMap;

	const mappings = shiftMappings(sourceMap.mappings);

	return mappings === sourceMap.mappings
		? sourceMap
		: { ...sourceMap, mappings };
};

/**
 * @param {Result} result a loader result: content, an optional source map, extra info
 * @returns {Result} the result with a leading BOM removed from its content and its source map kept in sync
 */
const removeBOMFromResult = (result) => {
	const content = result[0];
	const withoutBOM = removeBOM(content);

	if (withoutBOM === content) return result;

	// Copied rather than rebuilt, so a result carrying no source map keeps its
	// arity.
	const adjusted = /** @type {Result} */ ([...result]);

	adjusted[0] = withoutBOM;

	if (adjusted[1]) adjusted[1] = adjustSourceMapForRemovedBOM(adjusted[1]);

	return adjusted;
};

module.exports.adjustSourceMapForRemovedBOM = adjustSourceMapForRemovedBOM;
module.exports.removeBOM = removeBOM;
module.exports.removeBOMFromResult = removeBOMFromResult;