Verify the legitimacy of the object

Question

Verify the legitimacy of the object

I'm encountering an issue while attempting to create a function that verifies the validity of an object.

const isPeriodValid = (period: Period | null): boolean => {
   return period && period.start && period.end;
} 

export interface Period {
   start: number,
   end: number
}

The error message from my IDE reads: TS2322: Type 'number | null' is not assignable to type 'boolean'. Type 'null' is not assignable to type 'boolean'. The suggested fix is to return: null | 0 | number

What could be causing this issue?

typescript

Answer 1

Answer №1

After evaluating both your Period type declaration and the logic of your function, it seems that you are currently returning either a number or null from the function. TypeScript is flagging an issue because you have declared that the function should return a boolean.

An easy fix is to explicitly convert the result of your evaluation to a boolean:

const isPeriodValid = (period: Period | null): boolean => {
   return !!(period && period.start && period.end);
} 

export interface Period {
   start: number,
   end: number
}

Alternatively, a more elegant solution would involve creating a type predicate:

const isPeriodValid = (period: Period | null): period is Period => {
   return period != null && 'start' in period && 'end' in period;
   // or, for strict type checking: return period !== null;
} 

export interface Period {
   start: number,
   end: number
}

This approach offers the added advantage of asserting that the period variable is indeed of the Period type if you need to use it later in your code.

Answer 2

After evaluating both your Period type declaration and the logic of your function, it seems that you are currently returning either a number or null from the function. TypeScript is flagging an issue because you have declared that the function should return a boolean.

An easy fix is to explicitly convert the result of your evaluation to a boolean:

const isPeriodValid = (period: Period | null): boolean => {
   return !!(period && period.start && period.end);
} 

export interface Period {
   start: number,
   end: number
}

Alternatively, a more elegant solution would involve creating a type predicate:

const isPeriodValid = (period: Period | null): period is Period => {
   return period != null && 'start' in period && 'end' in period;
   // or, for strict type checking: return period !== null;
} 

export interface Period {
   start: number,
   end: number
}

This approach offers the added advantage of asserting that the period variable is indeed of the Period type if you need to use it later in your code.

Verify the legitimacy of the object

Answer №1

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