Refining the Union Type of Sets and Arrays for the find function

Here is the data model representation:

A person's daily data structure

interface FetchDocument {
    Date : number,
    Hours : number,
}   

Grouping collection of these data points

type Data_Array = Set<FetchDocument> | FetchDocument[]

interface GroupDocument {
    name : string,
    Data : Data_Array,
   
}

Sample test data provided below

let fetchArray : GroupDocument[] = [
    {
        name : 'John',
        Data : [
            {
                Date : 13,
                'Hours' : 14
            },
            {
                Date : 12,
                'Hours' : 433
            }
        ] 
    }
]

An attempt to find specific data using the `find` method:

for(let i= 0; i < fetchArray.length ; i++){
    let obj = fetchArray[i].Data.find((obj : FetchDocument) => obj.Date ===  13 )
}

The compiler presents an error message due to the use of Set within Data_Array.

Error TS2339: Property 'find' does not exist on type 'Data_Array'.

Property 'find' does not exist on type 'Set'.

Various attempts have been made to resolve this issue including narrowing and re-assigning:

Narrowing

 if(Array.isArray(fetchArray[i].Data)){
        let obj = fetchArray[i].Data.find((obj : FetchDocument) => obj.Date ===  13 )
 }
 if(typeof(fetchArray[i].Data.find) === 'function'){
        let obj = fetchArray[i].Data.find((obj : FetchDocument) => obj.Date ===  13 )
    }
   if(fetchArray[i].Data instanceof Array){
        let obj = fetchArray[i].Data.find((obj : FetchDocument) => obj.Date ===  13 )
    }

Re-assigning to array

fetchArray[i].Data = [...fetchArray[i].Data] 
let obj = fetchArray[i].Data.find((obj : FetchDocument) => obj.Date ===  13 )
fetchArray[i].Data = Array.from(fetchArray[i].Data.values())
let obj = fetchArray[i].Data.find((obj : FetchDocument) => obj.Date ===  13 )

Despite these efforts, the error persists. Further solutions are sought to address this issue.

Playground link : Link

Set datatype usage aims at detecting duplicate entries, hence modifying it would necessitate adjustments in other sections of the codebase.

Answer №1

The reason for this behavior is known as Temporal Uncertainty. Typescript cannot guarantee that there are no external factors affecting the content of fetchArray during code execution.

Instead of forcibly specifying the type, it is advisable to store the value you want to verify in a separate variable once it has been validated.

for (let i = 0; i < fetchArray.length; i++){
    let arr = fetchArray[i].Data;
    if (arr instanceof Array) {
        let obj = arr.find((obj : FetchDocument) => obj.Date === 13)
    }
}

Answer №2

It may be necessary to utilize Type Assertions in this scenario. However, if the data is not in an array format, it could lead to runtime errors. Therefore, caution must be exercised to handle potential runtime errors.

One way to approach this is as follows:


for(let i= 0; i < fetchArray.length ; i++){
    // Assuming data is of type FetchDocument[] but keep in mind it could change at runtime
    const data = fetchArray[i].Data as FetchDocument[] 
    // Implement error handling as a precaution
    let obj = data.find((obj : FetchDocument) => obj.Date === 13 )
}

Demo

Updated

An alternative approach is to use the Array.from method without requiring additional if statements.

for(let i= 0; i < fetchArray.length ; i++){
    let obj = Array.from (fetchArray[i].Data)
     .find((obj : FetchDocument) => obj.Date === 13 )
}

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