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[Properties] waypoint_order [Description] If optimizeWaypoints was set to true, this field will contain the re-ordered permutation of the input waypoints. For example, if the input was: Origin: Los Angeles Waypoints: Dallas, Bangor, Phoenix Destination: New York and the optimized output was ordered as follows: Origin: Los Angeles Waypoints: Phoenix, Dallas, Bangor Destination: New York then this field will be an Array containing the values [2, 0, 1]. Note that the numbering of waypoints is zero-based. If any of the input waypoints has stopover set to false, this field will be empty, since route optimization is not available for such queries. [Type] 
Array
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Google Maps JavaScript API V3 Reference   |   Google Maps JavaScript API   |   Google Developers