react-transition-group/src/utils/SimpleSet.js

export default class SimpleSet {
  constructor() {
    this.v = [];
  }
  clear() {
    this.v.length = 0;
  }
  has(k) {
    return this.v.indexOf(k) !== -1;
  }
  add(k) {
    if (this.has(k)) return;
    this.v.push(k);
  }
  delete(k) {
    const idx = this.v.indexOf(k);
    if (idx === -1) return false;
    this.v.splice(idx, 1);
    return true;
  }
}