iceweasel-start-page/common/Dedupe.jsm

this.Dedupe = class Dedupe {
  constructor(createKey) {
    this.createKey = createKey || this.defaultCreateKey;
  }

  defaultCreateKey(item) {
    return item;
  }

  /**
   * Dedupe any number of grouped elements favoring those from earlier groups.
   *
   * @param {Array} groups Contains an arbitrary number of arrays of elements.
   * @returns {Array} A matching array of each provided group deduped.
   */
  group(...groups) {
    const globalKeys = new Set();
    const result = [];
    for (const values of groups) {
      const valueMap = new Map();
      for (const value of values) {
        const key = this.createKey(value);
        if (!globalKeys.has(key) && !valueMap.has(key)) {
          valueMap.set(key, value);
        }
      }
      result.push(valueMap);
      valueMap.forEach((value, key) => globalKeys.add(key));
    }
    return result.map(m => Array.from(m.values()));
  }
};

const EXPORTED_SYMBOLS = ["Dedupe"];