csp-util.js

#!/usr/bin/env node

const fs = require("fs");
const path = require("path");
const htmlparser = require("htmlparser2");
const replaceInFile = require("replace-in-file");

const TARGET_FOLDER = path.join(__dirname, "public");

const INPUT_FILE = path.join(TARGET_FOLDER, "index.html");
const OUTPUT_FILE = path.join(TARGET_FOLDER, "_headers");
const TOKEN_TO_REPLACE = "__REPLACE_ME__";

console.log("Copying the CSP for Netlify headers");

const cspContent = getCspContentFrom(INPUT_FILE);

updateNetlifyHeaderFile(cspContent, OUTPUT_FILE);

function getCspContentFrom(inputFile) {
	console.log(`Getting the CSP content from ${inputFile}`);
	try {
		const fileContents = fs.readFileSync(inputFile, { encoding: "utf-8" });

		let found = false;
		let retVal = "";

		const parser = new htmlparser.Parser(
			{
				onopentag: (name, attributes) => {
					if (name === "meta") {
						if (attributes["http-equiv"] && "Content-Security-Policy" === attributes["http-equiv"]) {
							console.log("Found the CSP content:", attributes["content"]);
							found = true;
							retVal = attributes["content"];
						}
					}
				},
			},
			{ decodeEntities: true }
		);

		parser.write(fileContents);

		if (found === false) {
			throw new Error(`Could not find the CSP`);
		}

		return retVal;
	} catch (err) {
		console.error("Could not retrieve the CSP content. Did you build first? Is the gatsby-config.js still correct?");
		throw err;
	}
}

function updateNetlifyHeaderFile(cspText, outputFile) {
	console.log(`Updating the CSP in the output file [${outputFile}]`);
	const replacementOptions = {
		files: outputFile,
		from: new RegExp(TOKEN_TO_REPLACE, "g"),
		to: cspText,
	};

	try {
		console.log(`Modifying Netlify's header file`);
		const changes = replaceInFile.sync(replacementOptions);
		if (changes && changes.length && changes.length > 0) {
			console.log(`Modified Netlify's headers file successfully`, changes.join(", "));
		} else {
			throw new Error(`Failed to find the expected token to replace: ${TOKEN_TO_REPLACE}`);
		}
	} catch (error) {
		console.error(`Failed to modify Netlify's header file`);
		throw error;
	}
}