Brief Summary : Dart Variable

This post may helpful for those who need quick remind of Dart grammar, or a programmer from a different language learning Dart.

Basic

Variables store references.

var

var language = 'dart';

• Mutable variable keyword

By Type

String language = 'dart';

• You can specify the type in order to declare variable.

final & const

final fruits = {
    1: 'apple',
    2: 'orange'
};
const animals = {
    1: 'dog',
    2: 'cat'
};

fruits[1] = 'banana'; // NO ERROR
animals[1] = 'monkey'; // ERROR

• There’re similar in sense that they can’t be changed once you assign a value to them.

The Difference Between “final” and “const” keyword

• final can’t be changed once you assign a value.
• const is also like that, but in advance, they require more rules
  • The value that you’re going to assign should be a compile-time constant
    • compile-time constant means a constant value that is already fixed by the time when dart compile process.
    • For example, String literals that dose not contain interpolated expression and expression must be one of these(null, numeric, string, or boolean)
• Also, after assigning a value, variables annotated with final can’t be reassigned, but it’s fields can.
• Unlike final, variables annotated with const can’t be reassigned, both itself ant it’s sub fields.

late

late String onlyKnowAfterInitialization;

onlyKnowAfterInitialization = 'hello world';

• Keyword that’s used in below cases.
  • You need to assign a value after you declare it.
  • You want to use lazy assignment.
    • For example, when you need to access this in order to assign a value.
• If the value of a variable annotated with late keyword needs to be computed, the computation will not happen unless the variable is actually used in somewhere.

Cheers!