openapi-typescript-codegen/src/utils/getOpenApiVersion.ts

export enum OpenApiVersion {
    V2 = 2,
    V3 = 3,
}

/**
 * Get the Open API specification version (V2 or V3). This generator only supports
 * version 2 and 3 of the specification, so we will alert the user if we encounter
 * an incompatible type. Or if the type is missing...
 * @param openApi The loaded spec (can be any object)
 */
export function getOpenApiVersion(openApi: any): OpenApiVersion | undefined {
    const info: any = openApi.swagger || openApi.openapi;
    if (info && typeof info === 'string') {
        const c: string = info.charAt(0);
        const v: number = Number.parseInt(c);
        if (v === OpenApiVersion.V2 || v === OpenApiVersion.V3) {
            return v as OpenApiVersion;
        }
    }
    throw new Error(`Unsupported Open API version: "${String(info)}"`);
}