Finally, I found a solution to this problem without reinstalling npm and I'm posting it because in future it will help someone, Most of the time this error occurs javascript heap went out of the memory. As the error says itself this is not a problem with npm. Only we have to do is
instead of,
npm run build -prod
extend the javascript memory by following,
node --max_old_space_size=4096 node_modules/@angular/cli/bin/ng build --prod