@fastly/js-compute/dist/compiler-steps/precompileRegexes.js

JavaScript SDK and CLI for building JavaScript applications on [Fastly Compute](https://www.fastly.com/products/edge-compute/serverless).

import { parse } from 'acorn';
import { simple as simpleWalk } from 'acorn-walk';
import regexpuc from 'regexpu-core';
// Compiler Step - Precompile regexes
// This step runs any time after bundling
export const precompileRegexesStep = {
    outFilename: '__fastly_precompiled_regexes.js',
    async fn(ctx, index) {
        await ctx.magicStringWriter(this.outFilename, async (magicString, source) => {
            // PRECOMPILE REGEXES
            // Emit a block of JavaScript that will pre-compile the regular expressions given.
            // As SpiderMonkey will intern regular expressions, duplicating them at the top
            // level and testing them with both an ascii and utf8 string should ensure that
            // they won't be re-compiled when run in the fetch handler.
            const PREAMBLE = `(function(){
  // Precompiled regular expressions
  const precompile = (r) => { r.exec('a'); r.exec('\\u1000'); };`;
            const POSTAMBLE = '})();';
            const ast = parse(source, {
                ecmaVersion: 'latest',
                sourceType: ctx.moduleMode ? 'module' : 'script',
            });
            const precompileCalls = [];
            simpleWalk(ast, {
                Literal(node) {
                    if (!node.regex)
                        return;
                    let transpiledPattern;
                    try {
                        transpiledPattern = regexpuc(node.regex.pattern, node.regex.flags, {
                            unicodePropertyEscapes: 'transform',
                        });
                    }
                    catch {
                        // swallow regex parse errors here to instead throw them at the engine level
                        // this then also avoids regex parser bugs being thrown unnecessarily
                        transpiledPattern = node.regex.pattern;
                    }
                    const transpiledRegex = `/${transpiledPattern}/${node.regex.flags}`;
                    precompileCalls.push(`precompile(${transpiledRegex});`);
                    magicString.overwrite(node.start, node.end, transpiledRegex);
                },
            });
            if (precompileCalls.length) {
                magicString.prepend(`${PREAMBLE}${precompileCalls.join('')}${POSTAMBLE}\n`);
            }
        });
        await ctx.maybeWriteDebugIntermediateFiles(`__${index + 1}_precompiled_regexes.js`);
    },
};