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Try not to do mutation of variables inside an apply! It is very very unlikely to work how you want. I would expect something like the following to build that array:

secretName_list = serviceAccountArray.map(ksa => {
    let resourceId : pulumi.Input<string> = "thx/".concat(ksa.name); //"namespace/shared-keystore-jks-ksa" ;
    let secrets: pulumi.Output<output.core.v1.ObjectReference[]> = k8s.core.v1.ServiceAccount.get("ksa-".concat(ksa.name),resourceId).secrets;
	secrets.apply(v => v[0].name);
 });

I ran the below code

var secretName_list = args.applicationSecrets.map(ksa => {
    let resourceId : pulumi.Input<string> = "thx/".concat(ksa.name); //"namespace/shared-keystore-jks-ksa" ;
    let secrets: pulumi.Output<output.core.v1.ObjectReference[]> = k8s.core.v1.ServiceAccount.get("ksa-".concat(ksa.name),resourceId).secrets;
    console.log("secrent name");
	secrets.apply(v => v[0].name);
})

console.log ("arraylist ",secretName_list);

It returns the array with undefined items looks like the apply executed later and array get populated with undefined value.

Oh sorry no I missed a return there. Switching languages all the time it's hard recalling the correct syntax without putting things through an IDE.

var secretName_list = args.applicationSecrets.map(ksa => {
    let resourceId : pulumi.Input<string> = "thx/".concat(ksa.name); //"namespace/shared-keystore-jks-ksa" ;
    let secrets: pulumi.Output<output.core.v1.ObjectReference[]> = k8s.core.v1.ServiceAccount.get("ksa-".concat(ksa.name),resourceId).secrets;
    console.log("secrent name");
	return secrets.apply(v => v[0].name);
})

Thanks Fraser but looks like it returns the function Output

arraylist  [
   OutputImpl {
     __pulumiOutput: true,
     resources: [Function (anonymous)],
     allResources: [Function (anonymous)],
     isKnown: Promise { <pending> },
     isSecret: Promise { <pending> },
     promise: [Function (anonymous)],
     toString: [Function (anonymous)],
     toJSON: [Function (anonymous)]
   },
   OutputImpl {
     __pulumiOutput: true,
     resources: [Function (anonymous)],
     allResources: [Function (anonymous)],
     isKnown: Promise { <pending> },
     isSecret: Promise { <pending> },
     promise: [Function (anonymous)],
     toString: [Function (anonymous)],
     toJSON: [Function (anonymous)]
   }
 ]

how would i get the value from it

You can't. There is no way to go from Output<T> to T, the only valid thing you can do with Outputs is pass them in as inputs to resources or export them as stack values.

Thanks Fraser, your solution work for me