What's the usage of three dots (...) in dart List?

Question

I have my code written like this but it gives an error saying:

Error: A value of type 'List' can't be assigned to a variable of type 'Widget'.

Column(
  children: [
    Question(
      questions[_questionIndex]['questionText'],
    ),
    ...(questions[_questionIndex]['answers'] as List<String>)
        .map((answer) {
      return Answer(_answerQuestion, answer);
    }).toList()
  ],
)

Answer 1

Dart 2.3 introduce Spread operator (…)

Reference link : https://medium.com/flutter-community/whats-new-in-dart-2-3-1a7050e2408d

    var a = [0,1,2,3,4];
    var b = [6,7,8,9];
    var c = [...a,5,...b];

    print(c);  // prints: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Answer 2

Dart 2.3 comes with the spread operator (...) and the null-aware spread operator (...?), which allows us to add multiple elements in Collections.

  List<String> values1 = ['one', 'two', 'three'];
  List<String> values2 = ['four', 'five', 'six'];
  var output = [...values1,...values2];
  print(output); // [one, two, three, four, five, six]

---

For Flutter, we can use this inside column

   Column(
          children: [
            ...values.map((value) {
              return Text(value);
            }),
          ],
        ),

Output:

Answer 3

var x = [20,30,40];
var list = [1,2,3,x];

// Using the spread operator, adds the items individually not as a list
var separatedList = [1,2,3,...x];

print(list); // length = 4
print(separatedList); // length = 6

Results

[1, 2, 3, [20, 30, 40]]
[1, 2, 3, 20, 30, 40]