# Headers

The `Headers` class represents a set of HTTP headers and provides methods and
properties for working with them. It is very similar to the standard
[Headers class](https://developer.mozilla.org/en-US/docs/Web/API/Headers)
used in web APIs.


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## Constructor

### constructor(init)
`constructor(init?: HeadersInit)`
Creates a new instance of the `Headers` class.
 * `init` (optional): An optional parameter that can be an instance of
    `Headers`, Web API `Headers`, an array of key-value pairs, or a record of
    header fields and values.


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## Methods


### append(name, value)
`append(name: string, value: string): void` \
Appends a new value onto an existing header's value, or adds the header if it
does not already exist.
 * `name` - The name of the header.
 * `value` - The value to append to the header.


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### has(name)
`has(name: string): boolean` \
Checks if a header with a specified name exists.
 * `name` - The name of the header to check.


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### get(name)
`get(name: string): string | null` \
Returns the first value of the specified header.
 * `name` - The name of the header.


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### set(name, value)
`set(name: string, value: string): void` \
Sets a new value for an existing header, or adds the header if it does
not already exist.
 * `name` - The name of the header.
 * `value` - The new value to set for the header.


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### delete(name)
`delete(name: string): void` \
Removes a header and its value from the set of headers.
 * `name` - The name of the header to delete.


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### toJSON()
`toJSON(): Record<string, string>` \
Returns a JSON representation of the headers.


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### toString()
`toString(): string` \
Returns a string representation of the headers in JSON format.


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### getSetCookie()
`getSetCookie(): string[]` \
Returns an array of values of the 'Set-Cookie' header.


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### forEach(callbackFn)
`forEach(callbackfn: (value: string, key: string, iterable: Headers) => void): void` \
Executes a provided function once per each header present in the set of headers.
 * `callbackfn` - A function to execute for each header.
 * `thisArg` - An optional value to use as `this` when executing the callback function.


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### keys()
`keys(): IterableIterator<string>` \
Returns an iterator of all the header names.


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### values()
`values(): IterableIterator<string>` \
Returns an iterator of all the header values.


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### entries()
`entries(): IterableIterator<[string, string]>` \
Returns an iterator of all the header names and values as key-value pairs.


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## Examples
The following examples, illustrate how to construct a `Headers` class, append
and retreive values from the object.

```typescript
import { Headers } from "sherpa-core";
const _headers = new Headers();

_headers.append("Content-Type", "application/json");
_headers.get("Content-Type"); // returns "application/json"
```