Recently, I encountered a situation where typescript seems to be incorrectly narrowing the given type.
(value: number[] | null) => {
if ((value?.length ?? 0) > 0) value[0];
};
Even though the condition will not be true if the value is null, in the if branch it still shows me an 'Object is possibly 'null'' error message.
Should this type be narrowed and why or why not?
If this behavior is expected, are there any convenient workarounds available?
Not the complete solution, but here's an explanation:
The reason this error occurred is due to the lack of a direct check for null/undefined in your code, and TypeScript isn't quite advanced enough to detect this on its own.
You have two options to address this issue:
1. More detailed but safer approach:
You can use a check similar to the following:
(value: number[] | null) => {
if (value == null && value.length > 0) value[0];
};
Alternatively, you can use typeguards like this:
(value: number[] | null) => {
if (!isNull(value) && value.length > 0) value[0];
};
function isNull(value: any): value is null {
return value == null;
}
2. Option for the confident individuals:
If you are certain about your code, you can inform TypeScript by using the ! notation like so:
(value: number[] | null) => {
if (value?.length ?? 0 > 0) value![0];
};
I hope this information proves helpful.
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