Null aware assignment of map entry - dart

Question

Is there an elegant way to write the following statement for a map?
String? img = json['image'];
if (img != null) {
json['image'] = 'https://example.com/ingredients_100x100/' + img;
}
Basically I want to append the image IFF it exists and ignore if it does not. This has the opposite effect of ??= where the assignment occurs only if the image is found to be null. Which is similar to putIfAbsent.
The best I can come up with is this:
String? img = json['image'];
json['image'] = img != null ? 'https://example.com/ingredients_100x100/' + img : null;
I just feel like this is the wrong way of writing for null safety as the word null appears twice in a line.

Answer 1

You could also use the update method of Map. It will throw an error if the key 'image' does not exist.
With the update method, you can also easily provide a default if the key image does not exist.
const baseImgUrl = 'https://example.com/ingredients_100x100/';
const defaultImgFilename = 'default.png';
Map<String, dynamic> updateImage(Map<String, dynamic> json) {
try {
return json
..update(
'image',
(filename) => '$baseImgUrl$filename',
);
} catch (_) { // key not in map
return json;
}
}
Map<String, dynamic> updateImageWithDefault(Map<String, dynamic> json) {
return json
..update(
'image',
(filename) => '$baseImgUrl$filename',
ifAbsent: () => '$baseImgUrl$defaultImgFilename',
);
}
void main() {
Map<String, dynamic> jsonWithoutImage = {'data': 123};
updateImage(jsonWithoutImage);
print('JSON without image: $jsonWithoutImage');
// JSON without image: {data: 123}
updateImageWithDefault(jsonWithoutImage);
print('JSON with default image: $jsonWithoutImage');
// JSON with default image: {data: 123, image: https://example.com/ingredients_100x100/default.png}
Map<String, dynamic> jsonWithImage = {'data': 123, 'image': 'apple.png'};
updateImage(jsonWithImage);
print('JSON with image: $jsonWithImage');
// JSON with image: {data: 123, image: https://example.com/ingredients_100x100/apple.png}
}