Check out example codes for "dart double question mark". It will help you in understanding the concepts better.

Code Example 1

// This means a equals b, but if b is null then a equals 'hello'.

String a = b ?? 'hello';


// This means if b is null then set it equal to hello. Otherwise, don't change it.
b ??= 'hello';

Code Example 2

// This means a equals b, but if b is null then a equals 'hello'.

String a = b ?? 'hello';

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