I'm using @googlemaps/google-maps-services-js 3.4.0.

The DirectionsStep type contains a steps property which should be DirectionsStep[], but the included typings in common.d.ts define it as just DirectionsStep. In the API responses it's clearly an array, and the comment on it also says that it's supposed to be an array. Also, it should be optional.

Unless I'm missing something, I believe this is what common.d.ts:1024 should be:

export interface DirectionsStep {
    /* ... */
    /**
     * contains detailed directions for walking or driving steps in transit directions.
     * Substeps are only available when `travel_mode` is set to "transit".
     * The inner `steps` array is of the same type as `steps`.
     */
-    steps: DirectionsStep;
+    steps?: DirectionsStep[];
}