String.raw and ES6 template strings

string-raw.js

let a = 1, b = 2, foo="foo", bar="bar", credentials: "insecure";

String.raw`http://foo.org/bar?a=${a}&b=${b}
    Content-Type: application/json
    X-Credentials: ${credentials}
    { "foo": ${foo},
      "bar": ${bar}}`; 
      
/* The code above produces the result
"http://foo.org/bar?a=1&b=2
    Content-Type: application/json
    X-Credentials: undefined
    { "foo": foo,
      "bar": bar}"
*/