Sketch — Count number of symbol instances in the selected page

count_symbols.js

var sketch = require('sketch/dom');
var UI = require('sketch/ui');

var document = sketch.getSelectedDocument();
var page = document.selectedPage;

var message;
var symbol_count = 0;

function countSymbols(layer) {
  if (layer.type === 'SymbolInstance') {
    symbol_count += 1;
  }

  if (layer.layers != undefined) {
    layer.layers.forEach(childLayer => {
      countSymbols(childLayer);
    });
  }
};

countSymbols(page);
message = 'Number of symbol instances: ' + symbol_count;

console.log(message);
UI.message(message);