I can't figure out how to get source maps working.

[trusktr]

Here's my gulpfile:

const gulp = require('gulp')
const babel = require('gulp-babel')
// const cached = require('gulp-cached')
const typescript = require('gulp-typescript')
const merge = require('merge-stream')
const babelConfig = require('./babel.config')
const tsConfig = require('./tsconfig.json')
const sourcemaps = require('gulp-sourcemaps')

function transpile() {
    return merge()
}

function watch(task) {
    return gulp.watch(
        ['src/**/*.{js,jsx,ts,tsx}', '!src/**/*test.{js,jsx,ts,tsx}'],
        { ignoreInitial: false },
        gulp.parallel(task)
    )
}

gulp.task('build-cjs', () => {
    const jsPipe = gulp
        .src(['src/**/*.{js,jsx}', '!src/**/*test.{js,jsx}'])
        // .pipe(cached('js')) // in watch mode, prevents rebuilding all files
        .pipe(sourcemaps.init())
        .pipe(babel(babelConfig))

        .pipe(
            babel({
                plugins: [
                    '@babel/plugin-transform-modules-commonjs',
                    'babel-plugin-add-module-exports',
                ],
            })
        )

        // TODO source maps
        .pipe(sourcemaps.write())
        .pipe(gulp.dest('./'))

    const tsPipe = gulp
        .src(['src/**/*.{ts,tsx}', '!src/**/*test.{ts,tsx}'])
        // .pipe(cached('ts')) // in watch mode, prevents rebuilding all files
        .pipe(sourcemaps.init())
        .pipe(typescript(tsConfig.compilerOptions))
        .pipe(babel(babelConfig))

        .pipe(
            babel({
                plugins: [
                    '@babel/plugin-transform-modules-commonjs',
                    'babel-plugin-add-module-exports',
                ],
            })
        )

        // TODO source maps
        .pipe(sourcemaps.write())
        .pipe(gulp.dest('./'))

    return merge(jsPipe, tsPipe)
})
gulp.task('watch-cjs', () => watch('build-cjs'))

There are no errors. An output file looks like this:

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.Box = exports.default = void 0;

var _Mesh = _interopRequireDefault(require("./Mesh"));

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

class Box extends _Mesh.default {}

exports.Box = exports.default = Box;
Box.defaultElementName = 'i-box';
Box.defaultBehaviors = {
  'box-geometry': initialBehaviors => {
    return !initialBehaviors.some(b => b.endsWith('-geometry'));
  },
  'phong-material': initialBehaviors => {
    return !initialBehaviors.some(b => b.endsWith('-material'));
  }
};

I think a sourceMapUrl should appear at the bottom, along with a sibling source map? What have I done wrong?