code: function placeLayer(state, layer, targetId) {
    const new_layers = state.layers.slice();
    const idx1 = getLayerIndexById(new_layers, layer.id);
    const idx2 = getLayerIndexById(new_layers, targetId);
    if (idx1 !== -1) {
      new_layers.splice(idx1, 1);
    }
    const newIndex = (targetId && idx2 !== -1) ? idx2 : new_layers.length;
    new_layers.splice(newIndex, 0, layer);
    return Object.assign({}, state, {
      layers: new_layers,
    }, incrementVersion(state.metadata, LAYER_VERSION_KEY));
  }
docs: Move a layer in the layers list.
  
  @param {Object} state The redux state.
  @param {Object} layer The layer object to be moved.
  @param {string} targetId The id of the layer
  at the position where layer will be moved.
  
  @returns {Object} The new state object.