gulpfile.js

'use strict';

const build = require('@microsoft/sp-build-web');

build.addSuppression(`Warning - [sass] The local CSS class 'ms-Grid' is not camelCase and will not be type-safe.`);

var getTasks = build.rig.getTasks;
build.rig.getTasks = function () {
  var result = getTasks.call(build.rig);

  result.set('serve', result.get('serve-deprecated'));

  return result;
};

build.configureWebpack.mergeConfig({
  additionalConfiguration: (generatedConfiguration) => {

    generatedConfiguration.module.rules.push(
      {
        test: /node_modules[\/\\]@?react-leaflet[\/\\].*.js$/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: [
              ['@babel/preset-env', { targets: "defaults" }]
            ],
            plugins: ['@babel/plugin-proposal-nullish-coalescing-operator']
          }
        }
      }
    );

    return generatedConfiguration;
  }
});

/* fast-serve */
const { addFastServe } = require("spfx-fast-serve-helpers");
addFastServe(build);
/* end of fast-serve */

build.initialize(require('gulp'));