sampleTerrainMostDetailed

Example:

// Query the terrain height of two Cartographic positions
const terrainProvider = Cesium.createWorldTerrain();
const positions = [
    Cesium.Cartographic.fromDegrees(86.925145, 27.988257),
    Cesium.Cartographic.fromDegrees(87.0, 28.0)
];
const promise = Cesium.sampleTerrainMostDetailed(terrainProvider, positions);
Cesium.when(promise, function(updatedPositions) {
    // positions[0].height and positions[1].height have been updated.
    // updatedPositions is just a reference to positions.
});