Set Default Parameter Values for a Function in Flutter

This is a short guide to default parameter values for a function in Flutter (and Dart as well).

Default function parameters allow formal parameters to be initialized with default values if no value is passed.

Syntax:

functionName({
  param1 = defaultValue1,
  param2 = defaultValue2,
  ...
  paramN = defaultValueN
}){
  /* */
}

If you forget the curly brackets, you’ll see the following error message:

Error: Non-optional parameters can't have a default value.
Try removing the default value or making the parameter optional.   

Example

The code:

// kindacode.com
// main.dart

import "package:flutter/foundation.dart";

void productInfo({
  String name = 'Unknown',
  double price = 0.00,
  String color = 'Blue',
  String description = 'This is a fictional product',
}) {
  if (kDebugMode) {
    print('Product Name: $name');
    print('Product price: $price');
    print('Product color: $color');
    print('Introduction: $description');
  }
}

void main() {
  // Only provide name
  productInfo(name: 'KindaCode.com');

  // Only provide name and color
  productInfo(name: 'A Novel', color: 'Amber');

  // All params are passed
  productInfo(
      name: 'A Phone',
      price: 99.99,
      color: 'Silver',
      description: 'A strong smartphone');
}

Output:

Product Name: KindaCode.com
Product price: 0.0
Product color: Blue
Introduction: This is a fictional product

Product Name: A Novel
Product price: 0.0
Product color: Amber
Introduction: This is a fictional product

Product Name: A Phone
Product price: 99.99
Product color: Silver
Introduction: A strong smartphone

That’s it. Happy coding, and have a nice day. If you’d like to learn more about Dart and Flutter, take a look at the following articles:

• Dart: Capitalize the First Letter of Each Word in a String
• Dart: Calculate the Sum of all Values in a Map
• Dart regex to validate US/CA phone numbers
• How to Iterate through a Map in Flutter & Dart
• Using GetX (Get) for Navigation and Routing in Flutter
• Flutter + Firebase Storage: Upload, Retrieve, and Delete files

You can also take a tour around our Flutter topic page or Dart topic page for the latest tutorials and examples.