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Destructuring

The second syntactic sugar that we will learn is destructuring.

There are two types of destructuring, array destructuring and object destructuring. Let’s discuss them one by one.

Array Destructuring

A common use case in JavaScript is to access item of an array at specific position, e.g.

function getPersons() {
  return ['Ali', 'Ahkau', 'Muthu'];
}

var persons = getPersons();

var person1 = persons[0]; // 'Ali'
var person2 = persons[1]; // 'Ahkau'
var person3 = persons[2]; // 'Muthu'

console.log(`person1: ${person1}`);
console.log(`person2: ${person2}`);
console.log(`person3: ${person3}`);

With array destructuring:

function getPersons() {
  return ['Ali', 'Ahkau', 'Muthu'];
}

var [person1, person2, person3] = getPersons();

console.log(`person1: ${person1}`);
console.log(`person2: ${person2}`);
console.log(`person3: ${person3}`);

The thing that may confuse people when first learn about destructuring is they may confuse the expression on the left hand side ([]) as “creating a new array”.

Instead, think of the left hand side as “describing the shape” of the result of right hand side.

The great thing about the destructuring is it is very visual, you can virtually do a visual mapping between the result and the assigned variables. Besides, it’s like a self documenting code, where it document the expected shape of the result of getPersons call.

Ignoring Some Item

function getPersons() {
  return ['Ali', 'Ahkau', 'Muthu'];
}

var persons = getPersons();

var person1 = persons[0]; // 'Ali'
var person3 = persons[2]; // 'Muthu'

log(`person1: ${person1}`);
log(`person3: ${person3}`);

With destructuring,

function getPersons() {
  return ['Ali', 'Ahkau', 'Muthu'];
}
var [person1, , person3] = getPersons();

log(`person1: ${person1}`);
log(`person3: ${person3}`);

…Rest

Sometimes, you want to get the first few items of an array, and group all the remaining items into separate array.

function getPersons() {
  return ['Ali', 'Ahkau', 'Muthu', 'Siti', 'Lily'];
}

var persons = getPersons();
var person1 = persons[0]; // 'Ali'
var person2 = persons[1]; // 'Ahkau'
var others = persons.slice(2); // ['Muthu', 'Siti', 'Lily']

log(`person1: ${person1}`);
log(`person2: ${person2}`);
log(`others: ${others}`);

With the ... (rest) syntax, you can do this:

function getPersons() {
  return ['Ali', 'Ahkau', 'Muthu', 'Siti', 'Lily'];
}

var [person1, person2, ...others] = getPersons();

log(`person1: ${person1}`);
log(`person2: ${person2}`);
log(`others: ${others}`);

With Default Value

Sometimes, while getting a value from an array, you may want to assign a default value:

function getPersons() {
  return ['Ali', 'Ahkau'];
}

var persons = getPersons();

var person1 = persons[0];
var person2 = persons[1];
var person3 = persons[2] || 'Unknown Person';

Same code* with destructuring:

function getPersons() {
  return ['Ali', 'Ahkau'];
}

var [person1, person2, person3 = 'Unknown Person'] = getPersons();

There is a slight difference between the two versions of code: the default value in destructuring only kicks in only if the value is undefined, if the value is falsy (0, null, or ""), the value will not get assigned but it would for the first version.

function getPersons() {
  return ['Ali', 'Ahkau', ''];
}

var persons = getPersons();

var oldPerson3 = persons[2] || 'Unknown Person';

var [person1, person2, person3 = 'Unknown Person'] = persons;

console.log(oldPerson3); // 'Unknown Person'
console.log(person3); // ''

You may ask, why only undefined?

That’s because in most cases, undefined value is usually a out-of-bound case or unassigned scenario, while others values are assigned intentionally.

var persons = ['Ali', 'Ahkau'];

persons[2]; // undefined, because out of bound

var person1 = 'A';
var person2 = 'B';
var person3;

var personsGroup2 = [person1, person2, person3];

personsGroup2[2]; // undefined, unassigned

Nested Array Destructuring

Sometimes the data structure can be nested array (array in array).

function getPersons() {
  var managers = ['Ali', 'Ahkau'];
  var staffs = ['Eric', 'Ricky'];
  return [managers, staffs];
}

var allGroups = getPersons();
var group1 = allGroups[0];
var group2 = allGroups[1];

var manager1 = group1[0];
var manager2 = group1[1];

var staff1 = group2[0];
var staff2 = group2[1];

We can use destructuring here too!

function getPersons() {
  var managers = ['Ali', 'Ahkau'];
  var staffs = ['Eric', 'Ricky'];
  return [managers, staffs];
}

var [
  [manager1, manager2], // destructure managers
  [staff1, staff2], // destructure staffs
] = getPersons();

With nested destructuring, you need to be careful as it may throw you error!

function getPersons() {
  var managers = ['Ali', 'Ahkau'];
  return [managers];
}

var [[manager1, manager2], [staff1, staff2]] = getPersons(); // Error!

That’s because when you try to do the inner destructuring, the code is trying to destructure undefined. It’s clearer if you translate the new syntax to the old syntax:

function getPersons() {
  var managers = ['Ali', 'Ahkau'];
  return [managers];
}

var allGroups = getPersons();
var group1 = allGroups[0]; // ['Ali', 'Ahkau']
var group2 = allGroups[1]; // undefined

// var staff1 = group2[0]; // Error!
// var staff2 = group2[1];

So what should we do? Use the default value.

function getPersons() {
  var managers = ['Ali', 'Ahkau'];
  return [managers];
}

var [[manager1, manager2], [staff1, staff2] = []] = getPersons();

Exercise

Rewrite the following code with destructuring.

function logTodosPlan(activities) {
  var firstActivity = activities[0];
  var secondActivity = activities[1];
  var thirdActivity = activities[2];
  var doLaters = activities.slice(3);
  console.log('I will start my day with ' + firstActivity);
  console.log('Then I will ' + secondActivity);
  console.log('Finally, I ' + thirdActivity);
  console.log('Activities that I will do later are: ' + doLaters.join(', '));
}

logTodosPlan(['eat', 'sleep', 'code', 'read', 'watch Avengers', 'get a gf/bf']);

function announceParticipants(event, participants) {
  var teachers = participants[0];
  var students = participants[1] || [];

  var firstTeacher = teachers[0] || 'N/A';
  var secondTeacher = teachers[1] || 'N/A';
  var firstStudent = students[0] || 'N/A';
  var secondStudent = students[1] || 'N/A';

  console.log('Announcing participants for ' + event);
  console.log('The first two teachers are: ' + firstTeacher + ' and ' + secondTeacher);
  console.log('The first two students are: ' + firstStudent + ' and ' + secondStudent);
}

announceParticipants('Kung Fu Competition', [
  ['Yoda', 'Master Sifu', 'Ip Man', 'Wong Fei Hong'],
  ['Luke Skywalker', 'Kungfu Panda', 'Bruce Lee'],
]);

announceParticipants('Hackathon', [['Kyle Simpsons'], ['Noob']]);

announceParticipants('Quidditch', [['Naruto', 'Lufee']]);

Object Destructuring

A more common use case in JavaScript is to get specific properties from a object.

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
    age: 29,
  };
}

var me = getPerson();
var myFirstName = me.firstName;
var myLastName = me.lastName;
var myAge = me.age;

console.log(`I am ${myFirstName} ${myLastName}, aged ${myAge}`);

With destructuring:

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
    age: 29,
  };
}

var {
  firstName: myFirstName, // get firstName property and assign it to myFirstName
  lastName: myLastName, // get lastName property and assign it to myLastName
  age: myAge, // get age property and assign it to myAge
} = getPerson();

console.log(`I am ${myFirstName} ${myLastName}, aged ${myAge}`);

Shorthand

In most cases, your variable name will be similar to the property name:

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
    age: 29,
  };
}

var me = getPerson();
var firstName = me.firstName;
var lastName = me.lastName;
var age = me.age;

console.log(`I am ${firstName} ${lastName}, aged ${age}`);

In that case, the destructuring syntax can be even shorter:

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
    age: 29,
  };
}

var { firstName, lastName, age } = getPerson();

console.log(`I am ${firstName} ${lastName}, aged ${age}`);

Ignoring Some Item

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
    age: 29,
  };
}

var { firstName, lastName } = getPerson(); // age is not destructured, thus will be ignored

console.log(`I am ${firstName} ${lastName}`);

…Rest

Similar to array where you can collect all the remaining items into an array, you can do something similar for object:

Old syntax:

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
    age: 29,
  };
}

var person = getPerson();
var firstName = person.firstName;
var otherDetails = Object.assign({}, person); // we need to create a new object by cloning it
delete otherDetails.firstName; // then we delete specific property on this new object

console.log(firstName); // 'Malcolm'
console.log(otherDetails); // { lastName: 'Kee', age: 29 }

With destructuring:

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
    age: 29,
  };
}

var { firstName, ...otherDetails } = getPerson();

console.log(firstName); // 'Malcolm'
console.log(otherDetails); // { lastName: 'Kee', age: 29 }

With Default Value

You can provide default value during destructuring.

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
  };
}

var person = getPerson();
var firstName = person.firstName || '';
var age = person.age || 18;

With new syntax:

function getPerson() {
  return {
    firstName: 'Malcolm',
    lastName: 'Kee',
  };
}

var { firstName = '', age = 18 } = getPerson();

// this is how you want to provide default while rename it
var { age: myAge = 18 } = getPerson();

Nested Object Destructuring

Similar to array, you may have nested object

function getPerson() {
  return {
    name: {
      firstName: 'Malcolm',
      lastName: 'Kee',
    },
    address: {
      street: '9, Jln Besar',
      state: 'KL',
    },
    age: 29,
  };
}

var person = getPerson();
var name = person.name;
var firstName = name.firstName;
var address = person.address;
var livingState = address.state;
var age = person.age;

console.log(`${firstName}, aged ${age}, lives in ${livingState}`);

With destructuring

function getPerson() {
  return {
    name: {
      firstName: 'Malcolm',
      lastName: 'Kee',
    },
    address: {
      street: '9, Jln Besar',
      state: 'KL',
    },
    age: 29,
  };
}

var {
  name: { firstName },
  address: { state: livingState },
  age,
} = getPerson();

console.log(`${firstName}, aged ${age}, lives in ${livingState}`);

Similar to nested object destructuring, you may need to provide default empty object to avoid error.

function getPerson() {
  return {
    name: {
      firstName: 'Malcolm',
      lastName: 'Kee',
    },
  };
}

var {
  name: { firstName },
  // address: { state } // this would cause error
} = getPerson();

To avoid that, do this

function getPerson() {
  return {
    name: {
      firstName: 'Malcolm',
      lastName: 'Kee',
    },
  };
}

var {
  name: { firstName },
  address: { state } = {},
} = getPerson();

Combining Object and Array Destructuring

Example for Array of Object

function getPersons() {
  return [
    { name: 'Spiderman', age: 18 },
    { name: 'Thanos', age: 2000 },
    { name: 'Ironman', age: 40 },
  ];
}

var [{ name: heroName, age: heroAge }, { name: villainName, age: villainAge }] = getPersons();

Example for Object containing Array

function getPerson() {
  return {
    name: 'Malcolm',
    friends: ['Mickey', 'Tom', 'Jerry'],
  };
}

var {
  name,
  friends: [firstFriend, secondFriend, thirdFriend],
} = getPerson();

Exercise

Rewrite the following code with destructuring.

function logPerson(person) {
  var name = person.name;
  var firstName = name.firstName;
  var lastName = name.lastName;
  var age = person.age;

  console.log(firstName + ' ' + lastName + ', aged ' + age);
}

var persons = [
  {
    name: { firstName: 'Malcolm', lastName: 'Kee' },
    age: 29,
  },
  {
    name: { firstName: 'Andy', lastName: 'Lau' },
    age: 55,
  },
];

logPerson(persons[0]);
logPerson(persons[1]);

function getStyles(styles) {
  var stylesVal = styles || {};
  var display = stylesVal.display || 'block';
  var dimensions = stylesVal.dimensions || {};
  var width = dimensions.width || '100%';

  return {
    display,
    width,
  };
}

console.log(getStyles({ display: 'inline', dimensions: { width: '50px' } }));
console.log(getStyles());

Destructuring in Function Parameter

All the destructuring that we have discussed can be applied to function parameters.

function calculateTotal(data) {
  var price = data.price;
  var tax = data.tax || 6;
  return price * (1 + tax / 100);
}

function calculateTotal({ price, tax = 6 }) {
  return price * (1 + tax / 100);
}

In fact, the ... syntax has special meaning as function parameters: it converts the parameters into an array.

function addAll(...allNumbers) {
  // all the parameters becomes an item of allNumbers array
  var sum = 0;
  for (var x = 0; x < allNumbers.length; x++) {
    sum += allNumbers[x];
  }
  return sum;
}

addAll(2); // 2;
addAll(2, 3); // 5;
addAll(2, 3, 4); // 9;

Default Parameter for Function

In addition of concept of destructuring and rest could be applied to function parameter, the concept of default value applies to function parameter too.

function formatMoney(value, symbol = 'RM') {
  return `${symbol} ${value}`;
}

formatMoney(100, '$'); // "$ 100"
formatMoney(100); // "RM 100"
formatMoney(100, null); // "null 2"

formatMoney is a function that will join the value of a number with the currency symbol. However, the currency symbol (second parameter) will be default to 'RM' if not provided.

Similar to default value during destructuring, function default parameter only works if that parameter is undefined, as demonstrated in the third usage example.

Exercise

Rewrite the function to use destructuring to assign default

function formatNumber(number, options) {
  var actualOptions = options || {};
  var prefix = actualOptions.prefix || 'RM ';
  var suffix = actualOptions.suffix || '.00';
  return prefix + number + suffix;
}

console.log(formatNumber(100));
console.log(formatNumber(2, { prefix: '$ ' }));
console.log(formatNumber(2, { prefix: 'DKK ', suffix: ',00' }));