Lexical Closures :

• A closure is a function object that has access to variables in its lexical scope, even when the function is used outside of its original scope.

• Functions can close over variables defined in surrounding scopes. In the following example, makeAdder() captures the variable addBy. Wherever the returned function goes, it remembers addBy.

  dart

•   /// Returns a function that adds [addBy] to the
    /// function's argument.
    Function makeAdder(int addBy) {
      return (int i) => addBy + i;
    }
  
    void main() {
      // Create a function that adds 2.
      var add2 = makeAdder(2);
  
      // Create a function that adds 4.
      var add4 = makeAdder(4);
  
      assert(add2(3) == 5);
      assert(add4(3) == 7);
    }
  

Lexical Scope

• Dart is a lexically scoped language, which means that the scope of variables is determined statically, simply by the layout of the code. You can "follow the curly braces outwards" to see if a variable is in scope.

•   bool topLevel = true;
  
    void main() {
      var insideMain = true;
  
      void myFunction() {
        var insideFunction = true;
  
        void nestedFunction() {
          var insideNestedFunction = true;
  
          assert(topLevel);
          assert(insideMain);
          assert(insideFunction);
          assert(insideNestedFunction);
        }
      }
    }