At Entria we have a lot of Node.js scripts to automate common tasks
we also make a script for any migration and to make it easy to run a single migration for test purposes and each script starts with anonymous async functions that is auto executed.
Like this template

construn=async()=>{//migration code}(async()=>{try{awaitrun();}catch(err){// eslint-disable-next-lineconsole.log('err:',err);process.exit(1);}process.exit(0);})();

This works well, but not enough for our use case. Because we create tests for our scripts and migrations if they are used in production.

If you import the run function in your test files it will run the async functions in your tests, which is not the behavior you wanna. So we have a check async function that is only auto executed when we are running directly.

To make this check, we usedmodule.parent propriety, but it will be deprecated on Node v16.

Context of Node v14 and v16

In October 26th, 2021 Node.js v16 replaced v14 as the LTS release.
And with these changes we on Entria bring to us breaking changes into our codebase on Entria, like a module.parent.

module.parent has used on Node v14 to locate if script is a module or executable, like:

if(!module.parent){// running like `node myfile.js`}else{// running like require('./myfile.js')}

We had 206 files that usemodule.parent

And we want changes all occurrences ofmodule.parent torequire.main, that allows we check the same thing ofmodule.parent.

if(require.main===module){/// running like `node myfile.js`}else{// running like require('./myfile.js')}

To change all occurrences ofmodule.parent we used a codemod, with jscodeshift. Codemod is a tool/library to assist our with large-scale codebase refactors that can be partially automated.

But Eduardo, why you not use find and replace of your IDE?

R: Because this require a lot of attention and time of our developers, and if we not used codemod can't sure that can exists more module.parent on the future.

Time of code

We want change

if(!module.parent){}

to

if(require.main===module){}

How ?

We usedjscodeshift to codemod.

First you should add jscodeshift in your node_modules with

npm install -g jscodeshift

After this, you should create an archive that contains our codemod, in this case,replace-module-parent.js.

First, we should create a function that is used in all files of the folder that we select, and pass two arguments,fileInfo andapi.

ThefileInfo argument represents information of the currently processed file, andapi is object that exposes thejscodeshift library and helper functions from the runner.

// replace-module-parent.jsfunctiontransform(fileInfo,api){};module.exports=transform;

Now we want to get jscodeshift helpers, fromapi.jscodeshift and transform our code to AST (Abstract System Types).
And you can explore more of our AST hereAST Explorer.

constj=api.jscodeshift;constroot=j(fileInfo.source)

Now, we want find all occurrences ofif(!module.parent), and replace toif(require.main === module)

// finding all ocurrences of if(!module.parent)root.find(j.IfStatement,{type:'IfStatement',test:{type:'UnaryExpression',operator:'!',argument:{type:'MemberExpression',object:{type:'Identifier',name:'module'},property:{type:'Identifier',name:'parent'}}}}).filter((path)=>{if(path.node.test.type!=='UnaryExpression'){returnfalse;}returntrue;})

Replacing all torequire.main

.forEach((path)=>{constrequireMain=j.ifStatement(j.binaryExpression('===',j.memberExpression(j.identifier('require'),j.identifier('main')),j.identifier('module')),path.node.consequent,path.node.alternate)j(path).replaceWith(requireMain)});returnroot.toSource();

And on final our codemod is

functiontransform(fileInfo,api){constj=api.jscodeshift;constroot=j(fileInfo.source)root.find(j.IfStatement,{type:'IfStatement',test:{type:'UnaryExpression',operator:'!',argument:{type:'MemberExpression',object:{type:'Identifier',name:'module'},property:{type:'Identifier',name:'parent'}}}}).filter((path)=>{if(path.node.test.type!=='UnaryExpression'){returnfalse;}returntrue;}).forEach((path)=>{constrequireMain=j.ifStatement(j.binaryExpression('===',j.memberExpression(j.identifier('require'),j.identifier('main')),j.identifier('module')),path.node.consequent,path.node.alternate)j(path).replaceWith(requireMain)});returnroot.toSource();};module.exports=transform;module.exports.parser='ts';

To run this code you can use this on your terminal:

jscodeshift -t replace-module-parent.js [your-input-files] -d -p