Deep Copy

use JSON.parse , JSON.stringify

function changeAgePure(person) {
    var newPersonObj = JSON.parse(JSON.stringify(person));
    newPersonObj.age = 25;
    return newPersonObj;
}
    
var alex = {
    name: 'Alex',
    age: 30
};
var alexChanged = changeAgePure(alex);

console.log(alex); // -> { name: 'Alex', age: 30 }
console.log(alexChanged); // -> { name: 'Alex', age: 25 }

or use $.extend()

function changeAgePure(person) {
    var newPersonObj = $.extend(true, {}, person);
    newPersonObj.age = 25;
    return newPersonObj
}
    
var alex = {
    name: 'Alex',
    age: 30
};
var alexChanged = changeAgePure(alex);

console.log(alex); // -> { name: 'Alex', age: 30 }
console.log(alexChanged); // -> { name: 'Alex', age: 25 }

or use destructuring (ES6)

function changeAgePure(person) {
    var newPersonObj = { ...person };
    newPersonObj.age = 25;
    return newPersonObj
}
    
var alex = {
    name: 'Alex',
    age: 30
};
var alexChanged = changeAgePure(alex);

console.log(alex); // -> { name: 'Alex', age: 30 }
console.log(alexChanged); // -> { name: 'Alex', age: 25 }

🙏 Reference