Shuhei Kagawa

main, jsnext:main and module

@2017-01-05 00:00 - JavaScript

Node module's package.json has main property. It's the entry point of a package, which is exported when a client requires the package.

Recently, I got an issue on one of my popular GitHub repos, material-colors. It claimed that "colors.es2015.js const not supported in older browser (Safari 9)", which looked pretty obvious to me. ES2015 is a new spec. Why do older browsers support it?

I totally forgot about it at the time, but the colors.es2015.js was exposed as the npm package's jsnext:main. And to my surprise, it turned out that jsnext:main shouldn't have jsnext or ES2015+ features like const, arrow function and class. What a contradiction!


Module bundlers that utilizes tree shaking to reduce bundle size, like Rollup and Webpack 2, require packages to expose ES Modules with import and export. So they invented a non-standard property called jsnext:main.

However, it had a problem. If the file specified jsnext:main contains ES2015+ features, it won't run without transpilation on browsers that don't support those features. But normally people don't transpile packages in node_modules, and many issues were created on GitHub. To solve the problem, people concluded that jsnext:main shouldn't have ES2015+ features other than import and export. What an irony.


Now the name jsnext:main is too confusing. I was confused at least. People discussed for a better name, and module came out that supersedes jsnext:main. And it might be standardized.


I looked into a couple of popular repos, and they had both of jsnext:main and module in addition to main.

At this time, it seems to be a good idea to have both of them if you want to support tree shaking. If you don't, just go with only the plain old main.