Mutable version

medium-mutable-sample.js

let users = [
    {
        name: {
            first: 'John',
            last: 'Galt'
        },
        email: 'john.galt@whois.com',
        age: 30
    },
    {
        name: {
            first: 'Mickey',
            last: 'Mouse'
        },
        email: 'mickey.mouse@didney.com',
        age: 90
    },
    {
        name: {
            first: 'Charlie',
            last: 'Chaplin'
        },
        email: 'charlie.chaplin@hollywood.com',
        age: 129
    },
];

const ChangeUserFirstName = 'John',
    ChangeUserLastName = 'Galt',
    ChangeAge = 38;

// Following change spoils the existing object for everyone. Other users won't 
// have any way of knowing the original state.
users.forEach(user => {
    if (user.name.first === ChangeUserFirstName &&
        user.name.last === ChangeUserLastName) {
        user.age = 38;
    }
});

console.log('users', users);

// Find the changed record.
const changedRecord = users.filter(user => (user.name.first === ChangeUserFirstName &&
    user.name.last === ChangeUserLastName));
console.log('changedRecord', changedRecord);
// No way to retrieve original record now :(
// const originalRecord = users.filter(user=>user.name.first === ChangeUserFirstName &&
//         user.name.last === ChangeUserLastName);
// console.log('originalRecord', originalRecord);
// returns same record as changedRecord

// ============Output============
//   npx babel-node --presets @babel/preset-env mutability.js
// users [ { name: { first: 'John', last: 'Galt' },
//     email: 'john.galt@whois.com',
//     age: 38 },
//   { name: { first: 'Mickey', last: 'Mouse' },
//     email: 'mickey.mouse@didney.com',
//     age: 90 },
//   { name: { first: 'Charlie', last: 'Chaplin' },
//     email: 'charlie.chaplin@hollywood.com',
//     age: 129 } ]
// changedRecord [ { name: { first: 'John', last: 'Galt' },
//     email: 'john.galt@whois.com',
//     age: 38 } ]