Typescript Syntax in Apollo server

Question

As you know in appollo server you can define the server's schema by passing a string into gql.

const typeDefs = gql`
type Query {
  getBtcRates: [BtcRate]
}
`'

But what is gql? Is it a function or a method?

It's definition

export const gql: (
  template: TemplateStringsArray | string,
  ...substitutions: any[]
) => DocumentNode = gqlTag;

To me it looks more like a function, but this syntax is unknown to me, so wonder what exactly it is and why this is written this way.

Answer 1

gql is using syntax called tagged templates and is not TypeScript specific. For another example, styled-components also relies on this syntax.

From the docs:

Tags allow you to parse template literals with a function. The first argument of a tag function contains an array of string values. The remaining arguments are related to the expressions.

A basic example of how this works:

 var variable = 'world'; function myTag(strings, exp) { var str0 = strings[0]; // "Hello " var str1 = strings[1]; // ";" return `${str0}${exp}${str1}`; } var output = myTag`Hello ${ variable }.`; console.log(output); // Hello world!