The npm maintainers have rolled back the changes to the npm self-signed certificate. See More help with SELFSIGNEDCERTINCHAIN and npm. I'm leaving this ProTip available in the event npm publishes this certificate change again.
The npm maintainers announced on February 27th that npm’s Self-Signed Certificate is No More:
A bunch of users received a “SELFSIGNEDCERTINCHAIN” error during installing and publishing packages throughout the day today.
The cause: npm no longer supports its self-signed certificates.
However, the recommended fix failed for me.
Linux / OS X:
export npm_config_strict-ssl=false npm install npm -g --ca=null npm --version unset npm_config_strict-ssl
SETLOCAL SET npm_config_strict-ssl=false npm install npm -g --ca=null npm --version ENDLOCAL