TypeError: type.split is not a function

[itabbot]

Running the following code and then requesting 127.0.0.1:3000 throws an error: "TypeError: type.split is not a function".

const Koa = require("koa");
const app = new Koa();
app.use(async (ctx) => {
  const { response } = ctx;
  response.body = {};
  response.set("Content-Type", ["image/jpg", "application/json"]);
  console.log(response.type);
});
app.listen(3000);

Because the Content-Type header value is an array, not a string:

koa/lib/response.js

Lines 388 to 392 in 0c81a73

	get type () {
	const type = this.get('Content-Type')
	if (!type) return ''
	return type.split(';', 1)[0]
	},

Of course, the HTTP specification doesn't allow me to set multiple Content-Type headers, but both Node.js and Koa allow me to do so. When I set more than one Content-Type response header, I find that Chrome, Edge, Firefox, and Postman all automatically recognize the last one.

How should koa, as a framework, position itself?

Validate parameters against the HTTP specification (which Node.js doesn't do)? Something like this:

// koa/lib/response.js
set (field, val) {
  if (this.headerSent) return

  if (field.toLowerCase() === 'content-type') {
    assert(!Array.isArray(val), 'Multiple content types are not allowed')
  }

  // Other code...
}

Or, depending on the situation? Such as:

// koa/lib/response.js
get type() {
  // can be a string or an array
  const type = this.get("Content-Type");
  if (!type) return "";

  // When there are more than one type, get the last one
  return Array.isArray(type) ?
    type[type.length - 1].split(";", 1)[0] :
    type.split(";", 1)[0];
},