Do you want to request afeature or report abug?
Feature

What is the current behavior?
webpack searched for"sideEffects": false in package.json to perform the extra optimizations for esm modules. However iffalse is a string, the optimization isn't detected.

We should probably (in the case that package authors might mistake this as "false" vsfalse) put the wrong value in, and theres no simple way to detect (as a dev) why the 'deopt' occurs.
If the current behavior is a bug, please provide the steps to reproduce.

What is the expected behavior?
"sideEffects": "false" in package.json works the same as"sideEffects": false

If this is a feature request, what is motivation or use case for changing the behavior?
Motivation described above.

Please mention other relevant information such as the browser version, Node.js version, webpack version and Operating System.
webpack 4.0.0-alpha