Encountering a potential issue with strictNullChecks in TypeScript (v. 3.2.1):
interface IData {
data?: { payload: string };
}
const list: IData[] = [];
const index: number = 0;
//testing a
if (list[index].data)
list[index].data.payload = "a";
//testing b
const item = list[index].data;
if (item)
item.payload = "a";
Why does "test a" result in a warning "object is possibly undefined" while "test b" does not? Is there a specific rationale for this behavior?
The main distinction between the two code snippets lies in:
if (list[0].data)
list[0].data.payload = "a"; // Proceeds as expected
... and ...
if (list[index].data)
list[index].data.payload = "a"; // Potential NULL reference!
The reason for this variance can be attributed to the level of analysis required to reach a conclusion at compile time. An insightful discussion on GitHub regarding control flow analysis trade-offs sheds light on this topic.
In your scenario, utilizing the index variable impedes the effectiveness of the type guard. Employing an intermediary variable like 'item' serves as a practical workaround.
const item = list[index].data;
if (item)
item.payload = "a";
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