swc/scripts/sourcemap/decode.js

// Copied from https://gist.github.com/bengourley/c3c62e41c9b579ecc1d51e9d9eb8b9d2

//
// This program reads a sourcemap from stdin
// and replaces the "mappings" property with
// human readable content. It writes the output
// to stdout.
//
// 1. install the dependencies:
//    npm i concat-stream vlq
//
// 2. optional: install jq for pretty printing json
//
// 3. run the command like so:
//
//      cat my-source-map.js.map | node decode | jq .
//

const concat = require("concat-stream");
const vlq = require("./vlq");

const formatMappings = (mappings, sources, names) => {
    const vlqState = [0, 0, 0, 0, 0];
    return mappings.split(";").reduce((accum, line, i) => {
        accum[i + 1] = formatLine(line, vlqState, sources, names);
        vlqState[0] = 0;
        return accum;
    }, {});
};

const formatLine = (line, state, sources, names) => {
    const segs = line.split(",");
    return segs.map((seg) => {
        if (!seg) return "";
        const decoded = vlq.decode(seg);
        for (var i = 0; i < 5; i++) {
            state[i] =
                typeof decoded[i] === "number"
                    ? state[i] + decoded[i]
                    : state[i];
        }
        return formatSegment(...state.concat([sources, names]));
    });
};

const formatSegment = (
    col,
    source,
    sourceLine,
    sourceCol,
    name,
    sources,
    names
) =>
    `${col + 1} => ${sources[source]} ${sourceLine + 1}:${sourceCol + 1}${
        names[name] ? ` ${names[name]}` : ``
    }`;

process.stdin.pipe(
    concat((json) => {
        const map = JSON.parse(json);
        process.stdout.write(
            JSON.stringify({
                ...map,
                mappings: formatMappings(map.mappings, map.sources, map.names),
            })
        );
    })
);