const a = 1;
const b = 2;

const $ = require('gogocode');
const script = $(source);
// use $_$ as a wildcard to match AST element at any position you want
const aAssignment = script.find('const a = $_$');
// get matched AST element value
const aValue = aAssignment.match?.[0]?.[0]?.value;
// replace AST as same as replace a string
// but ignore code format (space、indent or linebreak)
script.replace('const b = $_$', `const b = ${aValue}`);
// generate ast to string
const outCode = script.generate();

const a = 1;
const b = 1;