Q:

webpack.validateSchema is not a function

Webpack are throwing this error all of a sudden:

TypeError: webpack.validateSchema is not a function

Everything was working fine Friday, not working today. No new commits to master since Friday.
Pruned NPM, that didn't work, deleted NPM folder and re-installed, no dice. Checked out to previous branches which have not been rebased from Master for over a week. Still the same.
Anyone have an idea?

A:

Looks like npm bug, since webpack-dev-server@2.1.0-beta.11 requires webpack@^2.1.0-beta.26 but npm failed to install it.
The easiest way to avoid the issue without updating too much is to change dependency in package.json to
  "webpack-dev-server": "2.1.0-beta.10",

Instead of something like
  "webpack-dev-server": "^2.1.0-beta.9",

"^" char before version says "compatible with". Removing it sticks to the version exactly.
Don't forget to run npm install or npm update afterwards.

A:

I ran into this problem today at virtually the same time as you, it turns out that webpack was updated again.
Here is what I did to fix it:
First I ran npm install and npm update to see what the result was.  I ran both of these commands because npm has a weird way of reporting unmet dependancies, sometimes its wrong and when you re-run the npm update or the npm install, you will realize that the unmet dependencies are no longer an issue.
After I ran these commands I noticed that the only remaining message was a warning:
npm WARN webpack-dev-server@2.1.0-beta.11 requires a peer of webpack@^2.1.0-beta.26 but none was installed.
To get rid of this I changed my package.json file to read "webpack": "2.1.0-beta.26" instead of "webpack": "2.1.0-beta.25" and ran another npm install.
After this I got another error when I tried running npm start which stated that there was a problem with my webpack config file.  In my case, I went to the webpack config file for my development environment (because I am not on production yet) and I found the culprit which was an invalid parameter called 'outputPath'. 
I commented out that line and now I get everything working fine.
Hope this helps, may just be a hack for now but hopefully it is a step in the right direction.
UPDATE:
Ok, so I was a bit wrong about everything 'working fine'.  It turns out that some of my loaders were not working correctly;  Bootstrap and some other things were not being loaded in properly, breaking my styles.  So, to get it back to where I was before, I deleted my node_modules folder and ran npm install using the following in package.json:
"webpack": "2.1.0-beta.25",
"webpack-dashboard": "^0.1.8",
"webpack-dev-middleware": "^1.6.1",
"webpack-dev-server": "2.1.0-beta.9",
"webpack-md5-hash": "^0.0.5",
"webpack-merge": "^0.15.0",

Hopefully discussions like this one will help us figure out how to move forward properly with the new versions of webpack being released.

A:

I got the same error as well.  I locked my version of webpack-dev-server in my package.json file and that prevented the error from occurring.  That doesn't fix the root problem of the bug though.
This is the version of webpack-dev-server that I'm using but I'm sure later versions work as well:
"webpack-dev-server": "2.1.0-beta.9",

