A more readable printout of the example at https://github.com/jamiebuilds/babel-handbook/blob/master/translations/en/plugin-handbook.md#babel-traverse

Babel Traverse Example.js

const parser = require('@babel/parser');
const traverse = require('@babel/traverse');

// console.log('parser is', parser);
// console.log('traverse is', traverse);

const code = `function square(n) {
  return n * n;
}`;

const ast = parser.parse(code);

logObjectWithMessage(ast, 'ast before traverse:');

traverse.default(ast, {
  enter(path) {
    // logObjectWithMessage(path, 'path is:');
    if (
      path.node.type === 'Identifier' &&
      path.node.name === 'n'
    ) path.node.name = 'x';
  }
});

logObjectWithMessage(ast, 'ast after traverse:');

function logObjectWithMessage(object, message) {
  removeProperties(object, ['start', 'end', 'loc']);
  console.log(message);
  console.log('='.repeat(message.length));
  console.dir(object, {depth: null});
}

/**
 * Recursively remove properties that:
 * - Are `null` or `undefined`
 * - Have 0 length
 * - Have been listed in the `properties` parameter
 *
 * @param {Object} object - The object to remove the properties from.
 * @param {String[]} properties - Names of properties to remove.
 */
function removeProperties(object, properties) {
  for (const property in object) {
    const value = object[property];

    if (value == undefined) delete object[property];
    else if (value.length === 0) delete object[property];
    else if (properties.includes(property)) delete object[property];
    else if (value instanceof Object) removeProperties(value, properties);
  }
}