Change all var to const

codemod_example_1.js

//Example: https://astexplorer.net/#/E25QuXone4

//test_1.js
var var1 = 'var';
//we want to change it to 
//const var1 = 'var';

//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();
};

codemod_example_2.js

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

var bar = 'bar';
bar = 'foo';

//Now we need to handle more complex scenario
//We need to check whether variables are reassigned or not.
export default function transformer(file, api) {
  const j = api.jscodeshift;
  const {expression, statement, statements} = j.template;

  return j(file.source)
    .find(j.VariableDeclaration, {
      kind: 'var',
    })
    .forEach(p=>{
      //TODO ....
      p.value.kind = 'const';
    	return p;
	  })
    .toSource();
};

//Actually it is too complex to write down to here.
//Please refer https://github.com/cpojer/js-codemod/blob/master/transforms/no-vars.js