# [Flutter] Function in Dart

2021-04-01

## Blog series

This blog post is a series. You can see the other posts on the link below.

## Outline

In this blog post, I will introduce how to use Function in Dart.

You can see the source code, that is introduced on this blog post, on the link below

## Function

We can define the `Function` and use it in Dart like below.

``````String add(int a, int b) {
int sum = a + b;
return 'Sum: \$sum';
}

void main() {
}
``````

## Optional parameter

When you define the Function, you can set the `Optional parameter` and set the default value on it like below.

``````String add(int a, int b, [int c = 0]) {
int sum = a + b + c;
return 'Sum: \$sum';
}

void main() {
}
``````

## Named parameter

When the Function has many parameters, we can’t remember the older of the parameters. In this case, we can use `Named parameter`. If you use Named parameter, you can set the parameter more clearly when you call the Function.

``````String add(int a, int b, {int c = 0, int d = 0}) {
int sum = a + b + c + d;
return 'Sum: \$sum';
}

void main() {
print(add(1, 1, c: 1, d: 1));
}
``````

## typedef

You can pass the Function via the parameter in Dart. However, if you don’t set the type of the parameter, you don’t know what value is passed, and how to use it.

In this case, you can use `typedef`. You can define `typedef` and you can set it on the Function parameter.

``````typedef Operator(int n, int m);

String add(int a, int b) {
int sum = a + b;
return 'Sum: \$sum';
}

String substract(int c, int d) {
int sub = c - d;
return 'Sub: \$sub';
}

String calculate(int x, int y, Operator op) {
return op(x, y);
}

void main() {
print(substract(2, 1));

print(op(2, 1));

op = substract;
print(op(2, 1));

print(calculate(2, 1, substract));
}
``````

## Complete

We’ve seen how to use Function in Dart to develop an app with Flutter. We can know there is no difference between Dart and other languages to define the Function and use it. Also, we’ve seen how to use `typedef` to define the type of the FUnction and set it for the parameter in the Function.

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## App promotion

You can use the applications that are created by this blog writer `Deku`.
`Deku` created the applications with Flutter.