JavaScript 报错？

js 代码如下

const esprima = require('esprima');
const estraverse = require('estraverse');

const code = `
  function add(a, b) {
    let c = a + b;
    return c;
  }
`;

const ast = esprima.parseScript(code);

const varMap = {
  'a': 'x',
  'b': 'y',
  'c': 'z'
};
estraverse.replace(ast, {
  enter(node) {
    if (node.type === 'Identifier' && varMap[node.name]) {
      node.name = varMap[node.name];
    }
  }
});

const newCode = escodegen.generate(ast);

console.log(newCode);

报错日志:ReferenceError: escodegen is not defined