Headers: append() method
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2017.
Note: This feature is available in Web Workers.
The append() method of the Headers
interface appends a new value onto an existing header inside a Headers
object, or adds the header if it does not already exist.
The difference between set() and append() is
that if the specified header already exists and accepts multiple values,
set() will overwrite the existing value with the new one, whereas
append() will append the new value onto the end of the set of values.
For security reasons, some headers can only be controlled by the user agent. These headers include the forbidden request headers and forbidden response header names.
Syntax
append(name, value)
Parameters
Return value
None (undefined).
Examples
Creating an empty Headers object is simple:
const myHeaders = new Headers(); // Currently empty
You could add a header to this using append():
myHeaders.append("Content-Type", "image/jpeg");
myHeaders.get("Content-Type"); // Returns 'image/jpeg'
If the specified header already exists, append() will change its value to
the specified value. If the specified header already exists and accepts multiple values,
append() will append the new value to the end of the value set:
myHeaders.append("Accept-Encoding", "deflate");
myHeaders.append("Accept-Encoding", "gzip");
myHeaders.get("Accept-Encoding"); // Returns 'deflate, gzip'
To overwrite the old value with a new one, use Headers.set.
Specifications
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| Fetch # ref-for-dom-headers-append① |
Browser compatibility
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