Don't rebuild webpack bundle if entry files didn't change

Question

Consider we have a webpack config like this:

entry: {
    app: './main.js',
    lib: './vendor.js',   
}

The vendor.js file only consists of a bunch of requires to libraries from node_modules. 99% of the time I build the bundle(s) the output lib.js bundle is exactly the same.

Can I somehow tell webpack that if the vendor.js file didn't change (or preferrably some other custom condition like checking the modification date of lib.js and package.json to detect if I possibly have new versions of modules in node_modules) I do not want to rebuild the lib.js bundle? It takes a substantial amount of time on my CI server because of typescript transpliations etc.

Answer 1

As far as I am aware Webpack only really knows if a file will be the same once it has already built it because so many factors can change the file contents. The modified date of a file really does not provide enough information to determine that it should not be built again so I'd advise against it or you'll probably end up breaking your builds at some point and leaving people confused.

However if you did feel the need to do this though you could if you wanted make your Webpack config dynamic and use fs.stat to read vendor.js and then only add it as an entry if its changed. Something roughly like this:

var fs = require('fs');

var config = {
    entry: {
        app: './main.js'
    }
    ...
};

var stats = fs.statSync('./vendor.js');
if (new Date(stats.mtime).getTime() > process.env.LAST_VENDOR_BUILD_TIMESTAMP) {
    config.lib = './vendor.js';
    // Then save new Date().getTime() somewhere like a DB and
    // pass it in as LAST_VENDOR_BUILD_TIMESTAMP on next build.
}

module.exports = config;

As you can see the only way to solve your problem is that each and every build needs to have knowledge about previous builds to achieve this. This is undesired as your builds should be discrete and not care about previous build results.

Alternatively you should also try excluding some node_modules from the build if it is taking a really long time. I've not built typescript projects before but I exclude all of node_modules and my builds run much faster. Other than that, you shouldn't really mind your CI server being a little slow, at least it will be robust.