Watch JSON formatting files and dynamically inject "$schema" property in a new generated file.

gulpfile.js

const { watch } = require('gulp');
const path = require('path');
const fs = require('fs');

const defaultColumnSchemaUrl = 'https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json';
const defaultViewSchemaUrl = 'https://developer.microsoft.com/json-schemas/sp/v2/view-formatting.schema.json';

exports.default = function () {
  const watcher = watch(['**/*.column{-,.}format.json', '**/*.view{-,.}format.json']);

  watcher.on('change', function (filePath) {
    const fileName = path.basename(filePath);
    const fileBasePath = path.dirname(filePath);
    let data = JSON.parse(fs.readFileSync(filePath));
    let index;

    if (fileName.indexOf('.column') != 0) {
      data = {
        '$schema': defaultColumnSchemaUrl,
        ...data
      };
      index = fileName.indexOf('.column');
    } else {
      data = {
        '$schema': defaultViewSchemaUrl,
        ...data
      };
      index = fileName.indexOf('.view');
    }

    const generatedName = fileName.substring(0, index) + ".json";
    fs.writeFileSync(path.join(fileBasePath, generatedName), JSON.stringify(data, null, 2));
  });
};