Map.get() returning undefined-CodePudding

I want to populate my map, memo, with viewID for the key and an array for the value. I want to concatenate onto the array if i encounter the same viewID in my loop, however typescript is telling me preconcat is possibly undefined. I thought using the has() method would avoid this but, still the same issue. Any help?

let memo:Map<string,{}[]> = new Map<string,{}[]> ();
    let total_row_count:number = 0;
    for(let i = 0; i < assetLoadQuery.length; i  ) {
        const arrayOfPerformanceResourceTiming = JSON.parse(assetLoadQuery[i].performanceResourceTimingJSON);
        const viewID:string = assetLoadQuery[i].viewID;
        if(memo.has(viewID)) {
            const preconcat = memo.get(viewID);
            const concat = preconcat.concat(arrayOfPerformanceResourceTiming);
            memo.set(viewID, concat);
        } else {
            memo.set(viewID, arrayOfPerformanceResourceTiming);
        }  
        total_row_count  ;
    }

   src/v1/metrics/asset-load.ts:50:28 - error TS2532: Object is possibly 'undefined'.

50             const concat = preconcat.concat(arrayOfPerformanceResourceTiming);
                              ~~~~~~~~~

CodePudding user response：

you could use the non-null assertion operator.

read the docs:

A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact. Specifically, the operation x! produces a value of the type of x with null and undefined excluded. Similar to type assertions of the forms x and x as T, the ! non-null assertion operator is simply removed in the emitted JavaScript code.

here is an example

CodePudding user response：

You have to assign what kind of array it is, otherwise you are just typing it as an empty array/tuple [], so an array with 0 elements:

const memo = new Map<string,any[]>()

Replace any with the type that it actually contains.