map (callback) {
      const object = this._ifNode(callback(this.object, 'object', this))
      const index = this.index
        ? this._ifNode(callback(this.index, 'index', this))
        : null
      const value = this._ifNode(callback(this.value, 'value', this))

      return new AssignmentNode(object, index, value)
    }
Create a new AssignmentNode whose children are the results of calling
the provided callback function for each child of the original node.
@param {function(child: Node, path: string, parent: Node): Node} callback
@returns {AssignmentNode} Returns a transformed copy of the node