map (callback) {
      const expr = this._ifNode(callback(this.expr, 'expr', this))

      return new FunctionAssignmentNode(this.name, this.params.slice(0), expr)
    }
Create a new FunctionAssignmentNode 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 {FunctionAssignmentNode} Returns a transformed copy of the node