/**
 * @license
 * Copyright Google LLC All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */
/**
 * Represents the header configuration options for an HTTP request.
 * Instances are immutable. Modifying methods return a cloned
 * instance with the change. The original object is never changed.
 *
 * @publicApi
 */
export declare class HttpHeaders {
    /**
     * Internal map of lowercase header names to values.
     */
    private headers;
    /**
     * Internal map of lowercased header names to the normalized
     * form of the name (the form seen first).
     */
    private normalizedNames;
    /**
     * Complete the lazy initialization of this object (needed before reading).
     */
    private lazyInit;
    /**
     * Queued updates to be materialized the next initialization.
     */
    private lazyUpdate;
    /**  Constructs a new HTTP header object with the given values.*/
    constructor(headers?: string | {
        [name: string]: string | number | (string | number)[];
    } | Headers);
    /**
     * Checks for existence of a given header.
     *
     * @param name The header name to check for existence.
     *
     * @returns True if the header exists, false otherwise.
     */
    has(name: string): boolean;
    /**
     * Retrieves the first value of a given header.
     *
     * @param name The header name.
     *
     * @returns The value string if the header exists, null otherwise
     */
    get(name: string): string | null;
    /**
     * Retrieves the names of the headers.
     *
     * @returns A list of header names.
     */
    keys(): string[];
    /**
     * Retrieves a list of values for a given header.
     *
     * @param name The header name from which to retrieve values.
     *
     * @returns A string of values if the header exists, null otherwise.
     */
    getAll(name: string): string[] | null;
    /**
     * Appends a new value to the existing set of values for a header
     * and returns them in a clone of the original instance.
     *
     * @param name The header name for which to append the values.
     * @param value The value to append.
     *
     * @returns A clone of the HTTP headers object with the value appended to the given header.
     */
    append(name: string, value: string | string[]): HttpHeaders;
    /**
     * Sets or modifies a value for a given header in a clone of the original instance.
     * If the header already exists, its value is replaced with the given value
     * in the returned object.
     *
     * @param name The header name.
     * @param value The value or values to set or override for the given header.
     *
     * @returns A clone of the HTTP headers object with the newly set header value.
     */
    set(name: string, value: string | string[]): HttpHeaders;
    /**
     * Deletes values for a given header in a clone of the original instance.
     *
     * @param name The header name.
     * @param value The value or values to delete for the given header.
     *
     * @returns A clone of the HTTP headers object with the given value deleted.
     */
    delete(name: string, value?: string | string[]): HttpHeaders;
    private maybeSetNormalizedName;
    private init;
    private copyFrom;
    private clone;
    private applyUpdate;
    private setHeaderEntries;
    /**
     * @internal
     */
    forEach(fn: (name: string, values: string[]) => void): void;
}