Headers: Headers() constructor

init Optional

An object containing any HTTP headers that you want to pre-populate your Headers object with. This can be a simple object literal with String values, an array of name-value pairs, where each pair is a 2-element string array; or an existing Headers object. In the last case, the new Headers object copies its data from the existing Headers object.

Creating an empty Headers object is simple:

const myHeaders = new Headers(); // Currently empty

You could add a header to this using Headers.append:

myHeaders.append("Content-Type", "image/jpeg");
myHeaders.get("Content-Type"); // Returns 'image/jpeg'

Or you can add the headers you want as the Headers object is created. In the following snippet we create a new Headers object, adding some headers by passing the constructor an init object as an argument:

const httpHeaders = {
  "Content-Type": "image/jpeg",
  "X-My-Custom-Header": "Zeke are cool",
};
const myHeaders = new Headers(httpHeaders);

You can now create another Headers object, passing it the first Headers object as its init object:

const secondHeadersObj = new Headers(myHeaders);
secondHeadersObj.get("Content-Type"); // Would return 'image/jpeg' — it inherits it from the first headers object

You can also add the headers you want as the Headers object is created by using a two-dimensional array to add multiple headers with the same values. In the following snippet we create a new Headers object with multiple Set-Cookie headers by passing the constructor an init array as an argument:

const headers = [
  ["Set-Cookie", "greeting=hello"],
  ["Set-Cookie", "name=world"],
];
const myHeaders = new Headers(headers);

• ServiceWorker API
• HTTP access control (CORS)
• HTTP