In dart optional positional parameters are defined within square braces [ ] in a function's parameter list. which means it will be optional that whenever we call function at that it is not mandatory to declare this variable which are in square backet , by default their value shown as null.
But , if we want to define this value then we can simple declare the value in function unlike named parameter we do not need to write named before the value for declare.
I have already write both example to learn easily.
void student(var name ,[ var roll , var age])
{
print("Name = $name");
print("Roll No = $roll");
print("Age = $age");
}
void main() {
student("Vinit");
}
//Name = Vinit
//Roll No = null
//Age = null
In this we only get value of name in function at the call time , so other both files are null shown in output .
While , in the below code we can see that when we call function I have pass all three value ,and It all the value are shown without any error even I did not write named parameter before the value.
void student(var name ,[ required var roll , var age]) { print("Name = $name"); print("Roll No = $roll"); print("Age = $age"); } void main() { student("Vinit" ,10 , 20); } //Name = Vinit //Roll No = 10 //Age = 20