?? mean in dart code example

Example 1: dart spread

arr1 = [1, 2, 3];
arr2 = [...arr1, 4, 5, 6]
// arr -> [1, 2, 3, 4, 5, 6]

Example 2: dart ?? operator

int val1;
final int val2 = 20;

console.log(val1 ?? val2); //result => 20 because val1 is null

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final int val1 = 30;
final int val2 = 20;

console.log(val1 ?? val2); //result => 30 because val1 is not null