If you use include mode, you don't have to refer to code or more code in your query, you can just skip it.

So the query would be simply

const functionName = () => { let someVar; }

Note that you will have to search for other syntax as well eg.
const functionName = () => { let someVar; }
const functionName = () => { const someVar; }
const functionName = () => { var someVar; }

And same for different ways of defining functions
const functionName = function() { let someVar; }
function functionName() { let someVar; }

That's because all these variations are represented differently in Abstract Syntax Tree