Generate a self-signed JWT token for use on GCP with Typescript

gcloud_bearer_token.ts

import { KJUR } from "jsrsasign";

function currentTimeInSeconds(): number {
	return Math.floor(Date.now() / 1000);
}

class Payload {
	iss: string;
	sub: string;
	email: string;
	aud: string;
	jti: string;
	target_audience: string;
	exp: number;
	iat: number;

	constructor(clientEmail: string, targetAudience: string, ttl: number) {
		const time = currentTimeInSeconds();
		this.iss = clientEmail;
		this.sub = clientEmail;
		this.email = clientEmail;
		this.aud = "https://www.googleapis.com/oauth2/v4/token";
		this.exp = time + ttl;
		this.jti = "jwt_nonce";
		this.iat = time;
		this.target_audience = targetAudience;
	}
}

export default class TokenProvider {
	idToken: string;
	ttl: number;
	serviceAcct: string;
	audience: string;
	privateKey: string;
	payload: Payload;

	constructor(serviceAcct: string, audience: string, privateKey: string) {
		this.ttl = 600;
		this.idToken = "";
		this.serviceAcct = serviceAcct;
		this.audience = audience;
		this.privateKey = privateKey;
		this.payload = new Payload(serviceAcct, audience, this.ttl);
	}

	async getIdToken(request: Request): Promise<string> {
		if (this.idToken == "" || this.expired()) {
			this.idToken = await this.generateToken(request);
		}
		return this.idToken;
	}

	private expired(): boolean {
		return currentTimeInSeconds() + 10 >= this.ttl;
	}

	private async generateToken(request: Request): Promise<string> {
		const header = {
			alg: "RS256",
			typ: "JWT",
			kid: this.serviceAcct,
		};

		const payload = new Payload(this.serviceAcct, this.audience, this.ttl);
		const jwt = KJUR.jws.JWS.sign(
			header.alg,
			JSON.stringify(header),
			JSON.stringify(payload),
			this.privateKey
		);
		const body =
			"grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion=" + jwt;

		const newRequestInit = {
			method: "POST",
			headers: {
				Authorization: `Bearer ${jwt}`,
				"Content-Type": "application/x-www-form-urlencoded",
				Accept: "application/json",
			},
			body: body,
		};

		const url = new URL("https://www.googleapis.com/oauth2/v4/token");
		const newRequest = new Request(request, newRequestInit);
		const tokenRequest = new Request(url, newRequest);

		return await fetch(tokenRequest)
			.then(function (response: Response) {
				return response.text();
			})
			.then(function (text) {
				return JSON.parse(text).id_token;
			});
	}
}