//test_1.js
var var1 = 'var';
const var2 = 'var2';

//start template
export default function transformer(file, api) {
  const j = api.jscodeshift;
  const {expression, statement, statements} = j.template;

  return j(file.source)
    //Using find to locate the code you want to modified
    //the first argument will be the type of targeting code.
    //the second arguments could be an object which is the subset of AST you want to find
    .find(j.VariableDeclaration, {
      kind: 'var',
    })
    //Using replaceWith or forEach to modified the source code you find
    //if you want to replace all node you find directly, use replaceWith directly.
    //However, if you only want to modify part of those code, or you want to further filter.
    //Then you could use forEach to execute more precise manipulation
    .forEach(p=>{
      p.value.kind = 'const';
    	return p;
	  })
    .toSource();
};