const sampleObject = {name : 'John', age : 34 }
const sampleArrayOfObject = [{name:'John',age:20}]
const updatedOject = {...sampleObject, age: sampleArrayOfObject[0].age}

I'm not exactly sure how you want the operation to be done, but here is one of the options:

function update(obj, objs) {
  return {
    ...obj,
    age: objs.find(v => v.name === obj.name)?.age ?? obj.age,
  };
}

let updatedOject = update(sampleObject, sampleArrayOfObject);

Here is playground for this option, and here is what you get when if you click "Run":

{
  "name": "John",
  "age": 20
} 

Here is the same option but with proper typings:

type Person = { name: string; age: number }

function update(obj: Person, objs: Person[]): Person {
  return {
    ...obj,
    age: objs.find(v => v.name === obj.name)?.age ?? obj.age,
  };
};

And this is the same code but with generics (I'd say a preferred approach, but don't take this for granted):

function update<T extends { name: string; age: number }>(obj: T, objs: T[]): T {
  return {
    ...obj,
    age: objs.find(v => v.name === obj.name)?.age ?? obj.age,
  };
};