Understanding Brackets in Dart (Flutter) - dart

Question

I have many confusion about Brackets in Dart(Flutter).
Which bracket "(), {}, []" is used for what?

Answer 1

() can group expressions:
var x = (1 + 2) * 3;
or can designate parameter lists for functions:
var getAnswer = () => 42;
int square(int x) => x * x;
or can designate function calls:
var answer = getAnswer();
var squared = square(4);
or is part of the syntax to some keywords. This includes (but is not limited to) if, assert, for, while, switch, catch:
if (condition) {
...
}
assert(condition);
for (var item in collection) {
...
}
while (condition) {
...
}
switch (value) {
...
}
try {
...
} on Exception catch (e) {
...
}
[] by itself creates List literals:
var list = [1, 2, 3];
var emptyList = []; // Creates a List<dynamic>.
or when used on an object, calls operator [], which usually accesses an element of a collection:
var element = list[index];
var value = map[key];
or in a parameter list, specifies optional positional parameters:
int function(int x, [int y, int z]) {
return x + y ?? 0 + z ?? 0;
}
function(1, 2);
or in dartdoc comments, creates linked references to other symbols:
/// Creates a [Bar] from a [Foo].
Bar fooToBar(Foo foo) {
// ...
}
{} can create a block of code, grouping lines together and limiting variable scope. This includes (but is not limited to) function bodies, class declarations, if-else blocks, try-catch blocks, for blocks, while blocks, switch blocks, etc.:
class Class {
int member;
}
void doNothing() {}
void function(bool condition) {
{
int x = 'Hello world!';
print(x);
}
if (condition) {
int x = 42; // No name collision due to separate scopes.
print(x);
}
}
or by itself can create Set or Map literals:
var set = {1, 2, 3};
var emptySet = <int>{};
var map = {'one': 1, 'two': 2, 'three': 3};
var emptyMap = {}; // Creates a Map<dynamic, dynamic>
or in a parameter list, specifies optional named parameters:
int function(int x, {int y, int z}) {
return x + y ?? 0 + z ?? 0;
}
function(1, y: 2);
or creates enumerations:
enum SomeEnumeration {
foo,
bar,
baz,
}
or in Strings is used to disambiguate interpolated expressions:
var foo = 'foo';
var foobar = '${foo}bar';
var s = '${function(1, 2)}';
<> when used as a pair in function or class declarations creates generics:
class GenericClass<T> {
T member;
}
T function<T>(T x) {
// ...
}
or specifies explicit types when using generics:
var map = <String, int>{};