Different behavior of (this) when we use function as constructor vs normal call

Question

I came across following scenario:

When I call function normally it can access contextObj

contextObj = {a: 22, b:55}
function user(){  console.log(this.contextObj.a)} 
user()

output = 22

but if I try following it gives me error undefined

contextObj = {a: 22, b:55}
function user(){  console.log(this.contextObj.a)} 
let obj = new user()

Can someone help me understand 'this'? is it because of keyword new

Thanks in advance :)