source: frontend/node_modules/agent-base/src/promisify.ts

import {
        Agent,
        ClientRequest,
        RequestOptions,
        AgentCallbackCallback,
        AgentCallbackPromise,
        AgentCallbackReturn
} from './index';

type LegacyCallback = (
        req: ClientRequest,
        opts: RequestOptions,
        fn: AgentCallbackCallback
) => void;

export default function promisify(fn: LegacyCallback): AgentCallbackPromise {
        return function(this: Agent, req: ClientRequest, opts: RequestOptions) {
                return new Promise((resolve, reject) => {
                        fn.call(
                                this,
                                req,
                                opts,
                                (err: Error | null | undefined, rtn?: AgentCallbackReturn) => {
                                        if (err) {
                                                reject(err);
                                        } else {
                                                resolve(rtn);
                                        }
                                }
                        );
                });
        };
}