Headers

`Headers.prototype.append`

Appends a new value onto an existing header inside a Headers object, or adds the header if it does not already exist

Syntax

JavaScript

headers.append(name, value)

Parameters

Parameter	Type	Description
name	string	The name of the HTTP header
value	string	The value to append

Return Value

undefined

Examples

Basic Usage

const headers = new Headers()
headers.append('Accept', 'text/html')
headers.append('Accept', 'application/json')
console.log(headers.get('Accept'))
// 'text/html, application/json'

Practical Example

function addCookies(headers: Headers, cookies: string[]) {
  cookies.forEach(c => headers.append('Set-Cookie', c))
}

Advanced Usage

const headers = new Headers()
headers.append('X-Tags', 'a')
headers.append('X-Tags', 'b')
console.log(headers.get('X-Tags')) // 'a, b'

Understanding Headers.prototype.append

The Headers.prototype.append method in JavaScript appends a new value onto an existing header inside a Headers object, or adds the header if it does not already exist. It belongs to the Headers object and is one of the most widely used methods for working with headers values in modern JavaScript and TypeScript applications.

The method signature is headers.append(name, value). It accepts 2 parameters: name, value. When called, it returns undefined. Understanding when and how to use append() helps you write more expressive, readable code.

Common use cases for Headers.prototype.append include data transformation, input validation, API response processing, and building reusable utility functions. It works well alongside related methods like fetch-headers-set, fetch-headers-get, fetch-headers-delete, enabling you to chain operations together for complex data manipulation pipelines.

Browser support for Headers.prototype.append is excellent across all modern browsers including Chrome, Firefox, Safari, and Edge. It is also fully supported in Node.js and Deno. For older environments, transpilation with Babel or a polyfill may be needed.