doevisualizations/doeVisualization/node_modules/d3-hierarchy/src/accessors.js

Data Visualization Library based on RequireJS and D3.js (v4+)

export function optional(f) {
  return f == null ? null : required(f);
}

export function required(f) {
  if (typeof f !== "function") throw new Error;
  return f;
}