constructor(sourceId, targetId, weight = 1) {
    this.id = null;
    this.sourceId = sourceId;
    this.targetId = targetId;
    this.weight = weight;
    this.bondType = '-';
    this.isPartOfAromaticRing = false;
    this.center = false;
    this.wedge = '';
  }
The constructor for the class Edge.

@param {Number} sourceId A vertex id.
@param {Number} targetId A vertex id.
@param {Number} [weight=1] The weight of the edge.