Javascript array that inside of map appears empty

Question

let arr1 =[{name:'mani',age:2},{name:'raj',age:2},{name:'s',age:1}];

let test = new Map();

for(const EI of arr1){

    if( !test.has(EI.age)){
        test.set(EI.age,new Array());
     
    }
    
    test.set(EI.age,test.get(EI.age).push(EI) );
     
}

when i console log this map, the array is empty for all the keys.

but it works as expected for the below code

let arr1 =[3,4,5,6,6,7,8,8,8];

let test = new Map();

for(const EI of arr1){

    if( !test.has(EI)){
        test.set(EI,1);
        continue;
    }

    test.set(EI,test.get(EI) + 1);

}

Even i tried taking copy of array test.set(EI,[test.get(EI)].push(1) ) still get empty array only

here the result is as expected

array is empty here .....

Answer 1

push returns the length, not the array.

so after this call:

test.set(EI.age,test.get(EI.age).push(EI) );

The map entry for EI.age isn’t an array. It’s a number.

And I would expect subsequent push calls for that entry to error.

And unless it’s your intention to skip the first occurrence you should remove the continue after creating the empty array.