A custom plugin for graphql-codegen to generate mergeable types

generate-apollo-mergeable-types.js

const { isInterfaceType, isObjectType } = require('graphql');

module.exports = {
  plugin: (schema, _documents, _config) => {
    const typesMap = schema.getTypeMap();

    let fileString =
      '/* This is an auto-generated file by generate-apollo-mergeable-types.js */\n\n';
    fileString += 'export const mergeableTypesArray = [\n';

    Object.keys(typesMap).forEach(typeName => {
      const type = typesMap[typeName];
      if (
        (!typeName.startsWith('__') && isObjectType(type)) ||
        isInterfaceType(type)
      ) {
        fileString += `  '${typeName}',\n`;
      }
    });

    fileString += '];\n';

    return fileString;
  },
};