[MOL-13183][SW] refactor fetching of oidc config to accommodate tests

Author: shawn wee

src/singpass/singpass-helper-ndi.ts

@@ -33,7 +33,7 @@ export class NdiOidcHelper {
 	private redirectUri: string;
 	private jweDecryptKey: Key;
 	private clientAssertionSignKey: Key;
-	private oidcConfig: Promise<OidcConfig>;
+	private oidcConfig: OidcConfig;
 
 	constructor(props: NdiOidcHelperConstructor) {
 		this.oidcConfigUrl = props.oidcConfigUrl;
@@ -46,8 +46,6 @@ export class NdiOidcHelper {
 			timeout: 10000,
 			proxy: props.proxyConfig,
 		});
-
-		this.oidcConfig = this.axiosClient.get<OidcConfig>(this.oidcConfigUrl).then((res) => res.data);
 	}
 
 	public constructAuthorizationUrl = async (
@@ -66,7 +64,7 @@ export class NdiOidcHelper {
 		userInfoScope: string[],
 		codeVerifier?: string,
 	): Promise<string> => {
-		const { authorization_endpoint } = await this.oidcConfig;
+		const { authorization_endpoint } = await this.getOidcConfig();
 
 		userInfoScope.unshift("openid");
 
@@ -90,7 +88,7 @@ export class NdiOidcHelper {
 	 * Use getIdTokenPayload on returned Token Response to get the token payload
 	 */
 	public getTokens = async (authCode: string, codeVerifier?: string): Promise<TokenResponse> => {
-		const { token_endpoint, issuer } = await this.oidcConfig;
+		const { token_endpoint, issuer } = await this.getOidcConfig();
 
 		const params = {
 			grant_type: "authorization_code",
@@ -149,8 +147,17 @@ export class NdiOidcHelper {
 		}
 	};
 
+	private getOidcConfig = async (): Promise<OidcConfig> => {
+		if (!this.oidcConfig) {
+			const response = await this.axiosClient.get<OidcConfig>(this.oidcConfigUrl);
+			this.oidcConfig = response.data;
+		}
+
+		return this.oidcConfig;
+	};
+
 	private getKeys = async () => {
-		const { jwks_uri } = await this.oidcConfig;
+		const { jwks_uri } = await this.getOidcConfig();
 		const {
 			data: { keys },
 		} = await this.axiosClient.get<{ keys: object[] }>(jwks_uri);