To install Node.js on CentOS 6, run following commands as user root

curl --silent --location https://rpm.nodesource.com/setup_6.x | sudo bash -
yum install -y nodejs

Once installed, you have node and npm available.

[root@s45-40-135-102 ~]# node --version
v6.11.2
[root@s45-40-135-102 ~]# npm --version
3.10.10
[root@s45-40-135-102 ~]# 

Now you can install required node modules with npm.