SOLVED: Npm support helped me track down the problem. Turns out that a script had changed my registry URL to something other than the official NPM registry.
Trying to publish a package https://github.com/glebec/batching-toposort.
npm publish succeeds, but it isn’t visible on the registry – https://registry.npmjs.org/batching-toposort shows a Not Found result. Trying to run
npm publish again errors, saying it cannot publish over an existing version.
From the shell,
npm info batching-toposort version prints
1.0.3 as expected. However,
npm info batching-toposort prints
npm ERR! Cannot read property '1.0.3' of undefined.
I thought the registry might just be lagging a bit, but I published this last night, and it has now been about 16 hours or so with no change.