Code becomes brittle if it uses information that's likely to change in the future. The more information it uses, the more brittle it becomes. For example, code that depends on checking strings in the user interface will break as soon as those strings change. Code that requies certain objects to be at certain indices in an Array will need to be fixed any time the indices change. Code that dives into an object structure to fetch information will fail whenever the structure changes. You should always be watching for brittle code. There's almost always a way to get the same effect without depending on details that are likely to change.
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