Exploring Sets and Maps in JavaScript

Exploring Sets and Maps in JavaScript

In this article, we’ve taken a look at Sets and Maps and how they handle unique items and key-value pairs respectively. These useful data structures provide easier and more efficient ways to structure and access data under certain use cases.

In this article, we’ve taken a look at Sets and Maps and how they handle unique items and key-value pairs respectively. These useful data structures provide easier and more efficient ways to structure and access data under certain use cases.

Let’s take~ ~a look into two new constructs that were introduced in the JavaScript ES6 specification:

Table of Contents

  • Sets
  • Maps
  • Conclusion
  1. *Set *- The Set object allows you to store unique values of any type.
  2. *Map *- The Map object allows you to store key-value pairs and remembers the original insertion order of the keys.

The objective of these new constructs is to provide easier and more efficient ways to structure and access data under certain use cases. In this article, we will look at how Sets and Maps work and explore some of the operations that can be performed on them.

Sets in JavaScript

The MDN docs describe the Set object as:

A collections of values. You can iterate through the elements of a set in insertion order. A value in the Set may only occur once; it is unique in the Sets collection.

The JavaScript Set object behaves similarly to the mathematical Set. It allows the addition of distinct values and provides useful methods on its prototype. These methods include - addition, removal and looping over of the items present in the Set.

Array vs Set

An array, like a set, is a data structure that allows addition, removal and looping operations on its items. However, an array differs from a set in the sense that it permits the addition of duplicate values and its operations are relatively slower.

Searching through an array has a linear time complexity of O(n), the same as inserting an element in the middle of an array. This means that the running time for searching and inserting items in an array grows as the size of the array increases.

JavaScript’s Push and Pop array methods have a run-time of O(1) which means that: these operations will have a constant time of execution regardless of the size of the array size. However, in practice, the Push operation is O(n) as copy costs are incurred when new contiguous memory locations are allocated to the newly formed array.

In contrast, all insert, delete and search operations for Sets have a running time of just O(1).

Creating a Set

Let’s create a Set:

const set = new Set();

console.log(set); // Set {}

Initializing a Set

To initialize a set, we can pass an array of values to the Set constructor, this will create a Set with those values:

const confectioneries = new Set(['oreo', 'marshmallow','oreo', 'kitkat', 'gingerbread']);

console.log(confectioneries); // result: Set { 'oreo', 'marshmallow', 'kitkat', 'gingerbread' }

In the snippet above, the duplicate value “oreo” is quietly removed from the Set and only unique values are returned.

Adding Items

We can add more items to a Set using the add() method. This method adds a new value to the Set object and returns the Set. An attempt to add a duplicate item to the Set object wouldn’t return an error, instead, the item will not be added.

Let’s go over an example:

const confectioneries = new Set(['oreo', 'marshmallow', 'kitkat', 'oreo','gingerbread']);

confectioneries.add('donut');

console.log(confectioneries); //_ log result: Set { 'oreo', 'marshmallow', 'kitkat', 'gingerbread', 'donut' } _

confectioneries.add('kitkat');

console.log(confectioneries); //_ log result: Set { 'oreo', 'marshmallow', 'kitkat', 'gingerbread', 'donut' } _

Deleting Items

With sets, we can delete items using either of these commands:

  • delete()
  • clear()

To use the delete() method, the value to be deleted is passed to the method. The method will return a Boolean value true if the deletion was successful and false if otherwise. We can delete all the elements of the Set object using the clear() method.

Let’s try out both methods in this example:

confectioneries.delete('kitkat');

console.log(confectioneries); //_ log result: Set { 'oreo', 'marshmallow', 'gingerbread', 'donut' }_

confectioneries.clear();

console.log(confectioneries); // log result: Set {}

Size of a Set

We can get the size of a Set using the size property on the Set prototype. This is similar to the length property for Arrays:

const confectioneries = new Set(['oreo', 'marshmallow', 'kitkat', 'oreo','gingerbread']);

console.log(confectioneries.size); // log result: 5

Searching for Items

We may need to know if a Set has a particular item. This can be accomplished using the has() method. The has() method returns true if the item is in the Set object, and false if it isn't:

console.log(confectioneries.has('marshmallow')); // log result: true


Returning the Items in a Set

We can return the items in a Set object in the same insertion order using the values()method. This method returns a new setIterator object . A similar method for returning the items of a set is the keys() method:

console.log(confectioneries.values()); // _log result: _[_Set Iterator] { 'oreo', 'marshmallow', 'kitkat', 'gingerbread', 'donut' }_

console.log(confectioneries.keys()); //_ log result: _[_Set Iterator] { 'oreo', 'marshmallow', 'kitkat', 'gingerbread', 'donut' }_

The setIterator object is an Iterator object because it implements the Iteratable and Iterator protocols. The Iterable protocol specifies a way to iterate through a set of values using loop constructs. It also makes it possible for the values to be iterated using the next() method. When we call next() on a setIterator object, we get the next value in the iteration and a false until all values of the Set have been iterated over:

let iterator = confectioneries.values();

console.log( iterator.next()); // _{ value: 'oreo', done: false } 
_
console.log( iterator.next()); // _{ value: 'marshmallow', done: false }
_
console.log( iterator.next()); //_ { value: 'kitkat', done: false }
_
console.log( iterator.next()); //_ { value: 'gingerbread', done: false }
_
console.log( iterator.next()); //_ { value: 'donut', done: false }
_
console.log( iterator.next()); // _{ value: undefined, done: true }_

Since Sets implement the Iterable protocol, loop constructs such as for ...ofcan be used as shown below:

for (let confectionery of confectioneries) {
  console.log(confectionery);
}

/_ _console.log() result 
oreo
marshmallow
kitkat
gingerbread
donut 
__/

WeakSets

WeakSets provide extra flexibility when working with the Set data structure. They are different from regular Sets in that they only accept objects and are not iterable; they can’t be looped over, and do not have a clear() method. How then do they provide extra flexibility? We’ll see in a bit.

We can create a WeakSetusing the WeakSet constructor:

let user1 = {name: 'user 1', email: '[email protected]'};
let user2 = {name: 'user 2', email: '[email protected]'};
let user3 = {name: 'user 3', email: '[email protected]'};

const users = new WeakSet([user1, user2, user3]);

The code above creates a new WeakSet object, adding items other than objects returns a TypeError:

users.add('user 4');

console.log(users); // TypeError: Invalid value used in weak set

Since WeakSets do not have a clear() method, objects can only be deleted by setting them to null. This works because the JavaScript Engine’s garbage collection algorithms will automatically free up memory allocated to the null object, hence deleting it from the WeakSet.

This is wonderful because the WeakSets objects set to null are garbage-collected while the program is still running, hence, reducing memory consumption and preventing memory leakage, especially when dealing with huge amounts of data that are generated asynchronously.

Within this feature lies a chance for you to write light-weight solutions to programming problems without having to bother with the details of memory management.

Maps In JavaScript

JavaScript Maps are objects designed to efficiently store and retrieve items based on a unique key for each item. A Map stores key-value pairs where both keys and values could be either primitive values or objects, or both.

The MDN docs describe the Map object as:

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value. A Map object iterates its elements in insertion order — a for...of loop returns an array of [key, value] for each iteration.

Creating a Map

Akin to Sets, Maps are easy to create. Let’s create a Map using the Map constructor:

const users = new Map();

console.log(users); // Map {}

Adding Items

Key-value pairs are added to a Map using the set() method. This method takes in two arguments, the first being the key and the second, the value, which is referenced by the key:

users.set('John Doe', {
  email: '[email protected]',
});

users.set('Jane Doe', {
  email: '[email protected]',
});

console.log(users);

/__ console.log result 
Map {
  'John Doe' => { email: '[email protected]'},
  'Jane Doe' => { email: '[email protected]'} }
__/

Unlike Sets which discard duplicate keys, Maps will update the value attached to that key:

users.set('John Doe', {
  email: '[email protected]',
});

console.log(users);

/__ console.log result 
Map {
  'John Doe' => {email: '[email protected]'},
  'Jane Doe' => { email: '[email protected]'} }
__/

When you run the example above, John Doe's email will neatly be replaced. Smooth.

Deleting Items

As with Sets, key-value pairs can be deleted using the delete() method. The key to be deleted is passed to the delete() method as shown below:

users.delete('Jane Doe');


Maps also have a clear() method, this removes all key-value pairs from the Map object:

users.clear();

console.log(users); // Map {}

Searching for Items

Maps also have a has() method which checks if a key exists in a Map. This method will return true if the key is in the Map and false if it is not:

let users = new Map();

users.set('John Doe', {
  email: '[email protected]',
});

users.set('Jane Doe', {
  email: '[email protected]',
});

console.log(users.has('John Doe')); // true

Returning the Value of a Map item

The value of a key in a Map object can be gotten using the get method on the Map prototype:

console.log(users.get('Jane Doe'); // { email: '[email protected]' }


It is possible to get all the keys and values of a Map object using the keys() and values() methods respectively. These methods both return a new MapIterator object which has a next() method that can be used to loop through the items of the Map:

let userKeys = users.keys();

console.log(userKeys.next()); // { value: 'John Doe', done: false }

let userValues = users.values();

console.log(userValues.next()); // _{ value: { email: '[email protected]' }, done: false }_

As with Sets, loop constructs such as for...of and forEach() can be used to loop through Map items:

for (let user of users) {
  console.log('[for...of]: ', user);
}

/_ Log result
  _[_for...of]:  _[_ 'John Doe', { email: '[email protected]' } ]
  _[_for...of]:  _[___ 'Jane Doe', { email: '[email protected]' } ]
_/

users.forEach((value, key) => console.log('[__forEach()]:  ', key, value));

/*_ Log result
  [__forEach()]:   John Doe { email: '[email protected]' }
  _[_forEach()]:   Jane Doe { email: '[email protected]' }
*_/

WeakMaps

As with WeakSets, WeakMaps differ from regular Map objects. WeakMaps only accept objects as keys, are not iterable and do not have a clear() method.

A WeakMap constructor is used to create a WeakMap.

Let’s look at an example:

let users = new WeakMap();

const user1 = {
  name: 'John Doe',
};
const user2 = {
  name: 'Jane Doe',
};

users.set(user1, {
  email: '[email protected]',
});

users.set(user2, {
  email: '[email protected]',
});

As with WeakSets, setting the key of a WeakMap object to null will implicitly garbage collect that item:

user1 = null;


This has the same advantages as with WeakSets in providing easier memory management.

Conclusion

In this article, we’ve taken a look at Sets and Maps and how they handle unique items and key-value pairs respectively. These useful data structures provide easier and more efficient ways to structure and access data under certain use cases.

Special modifications such as WeakSets and WeakMaps provide more options for the developer and are handy for memory management.

JavaScript ES6 Classes

JavaScript ES6 Classes

An exciting new construct that was introduced in the ES6 specification is the ES6 classes. If you're a Javascript developer, you will be aware that Javascript follows prototypal inheritance and sometimes it can get a little messy. However, with ES6 classes the syntax is simpler and much more intuitive.

An exciting new construct that was introduced in the ES6 specification is the ES6 classes. If you're a Javascript developer, you will be aware that Javascript follows prototypal inheritance and sometimes it can get a little messy. However, with ES6 classes the syntax is simpler and much more intuitive.

Classes are a fundamental part of object oriented programming (OOP). They define “blueprints” for real-world object modeling and organize code into logical, reusable parts. Classes share many similarities with regular objects. They have their own constructor functions, properties, and methods. In this tutorial, we’ll demonstrate how to create and work with ES6 classes.

Creating a class

You can create a class using the class keyword:

class Person{

  constructor(name, age) {

    this.name = name

    this.age = age

  }

}



let person = new Person("Sam", 30)



person.name

//returns 'Sam'



person.age

//returns 30

Notice how we define our Person class with a constructor() function taking two arguments name and age. Using the this keyword, we set the name and age properties based on the provided arguments. Remember that the constructor function is called whenever we create a new instance of the Person class.

Similar to objects, we can read class properties using dot notation so that person.name returns Sam.

Defining properties and methods

You can define properties and methods for a class the same way you do for regular objects:

class Person{

  constructor(name, age) {

    this.name = name

    this.age = age

  }



  sayName() {

    console.log("My name is " + this.name)

  }

}



let person = new Person('Tim', 40)



person.sayName()

//logs 'My name is Tim'



person.location = 'London'



person.location

//returns 'London'

Notice how we define a sayName() function within our Person class definition. Once we create a new instance of Person, we can call the method via person.sayName().

You can also add properties and methods on the fly. You’ll notice that while the location property isn’t defined in our constructor function, we can still dynamically add it later on for the person instance. Remember that if we created a new instance of Person, it would not have a location property because that property is not defined in the class definition. Only properties and methods that we explicitly define will be shared by all instances of the class.

Static functions

You can use the static keyword to make class methods static. A static method acts on the class itself, not on instances of the class:

class Person{

  constructor(name, age) {

    this.name = name

    this.age = age

  }



  static describe(){

    console.log("This is a person.")

  }



  sayName() {

    console.log("My name is " + this.name)

  }

}



Person.describe()

//logs 'This is a person.'

Notice how static methods operate on the class itself and not an instance of the class. We didn’t have to create a new Person to call the static method.

Static methods are useful for common or shared class functionality. In this case, the describe() method is used to describe what the Person class is. It will apply to every instance of Person. This is why we make it a static method.

Class Inheritance

Inheritance allows you to create new classes based off existing ones. These new classes “inherit” the methods and properties of their parent. They can also override or extend the parent:

class Person{

  constructor(name, age) {

    this.name = name

    this.age = age

  }


  static describe(){

    console.log("This is a person.")

  }


  sayName() {

    console.log("My name is " + this.name)

  }

}



class Programmer extends Person {

  sayName(){

    console.log("My name is " + this.name + " and I am a programmer!")

  }

}



let averageJoe = new Person('Todd', 40)

let programmer = new Programmer('Sam', 33)



averageJoe.sayName()


//logs 'My name is Todd'


programmer.sayName()


//logs 'My name is Sam and I am a programmer!'

Using the extends keyword, we can create a new class sharing the same characteristics as Person. Notice how we override the sayName() method with a new definition for the Programmer class. Apart from overriding this method, everything else remains the same for both Person and Programmer.

Using super

The super keyword allows a child class to invoke parent class properties and methods.

class Person{

  constructor(name, age) {

    this.name = name

    this.age = age

  }



  static describe(){

    console.log("This is a person.")

  }



  sayName() {

    console.log("My name is " + this.name)

  }

}





class Programmer extends Person {

  sayName(){

    super.sayName()

    console.log("My name is " + this.name + " and I am a programmer!")

  }

}



let averageJoe = new Person('Todd', 40)

let programmer = new Programmer('Sam', 33)



programmer.sayName()



//logs 'My name is Sam'

//logs 'My name is Sam and I am a programmer!'

Notice how we call super.sayName() in the Programmer implementation of sayName(). While this invokes the parent implementation of super.sayName(), the name property still references the Programmer class.

Conclusion

Classes facilitate object oriented programming in JavaScript. While regular objects provide similar functionality, classes provide the extra advantage of inheritance and static methods.

Linked Lists in JavaScript With ES6

Linked Lists in JavaScript With ES6

Linked Lists in JavaScript with ES6.This series is about data structure implementation in JavaScript using the ES6 specification.. Let's see how to make a singly linked list in Javascript… ... list in Javascript. We'll be using ES6 syntax throughout.

This is a continuation of a previous piece where we digested all surrounding concepts, pros and cons, Big O time complexity, real use cases, linked-list mainly operations, and all that kind of theory. If you have not read it yet, I recommend you read it first.

This series is about data structure implementation in JavaScript using the ES6 specification.

The aim of this second piece is to walk through the implementation of a linked list. Actually, the two pieces enclose a linked list itself since the prior piece is pointing to this one.

The Node Class

In the next code, we’re going to define our Node class with its constructor. Remember, the node is the basic building block to store the data and the next pointer.

This class will have to handle the node creation. Every time the class is instantiated, the constructor has the responsibility to initialize the two properties: data and next.


Node Class

Now the challenge is to create the next four nodes (just nodes creation, not how to connect them).


Linked List

Basically, we have to instantiate the Node class four times in order to create the four nodes.


Creating Nodes

At this point, we don’t care about the second parameter. Why? Because at this moment, we’re just learning how to create the node without having to worry about how they’ll be connecting together.

How Can We Connect the Nodes?

In the prior code, we just created nodes independently. Now is time to learn how to connect them to form the linked list.


Connecting nodes

We have defined the Node class. Next is to define a new class that will handle the nextpointer property and the main operations in the linked list. Let’s create the LinkedList class.


Linked List Class

In the above code, we have just defined a class called LinkedList with its constructor. This has the work of initializing the headproperty, to store the first node,and size, to keep track of the size of the linked list.

Next is to offer the ability to insert to the head, to the tail, or at any random position in the list.

Inserting Into the Head


Inserting to head

We have just created a simple method to add nodes to the head of the linked list. We are passing down to it the dataparameter and setting a value for the this.head property creating a new instance of the Node class.

Let’s do some tests of its implementation so far and see the results.

The output will be:


Linked list output

Inserting at the Tail

We just learned how to add nodes to the head. It’s time to know how to add nodes to the tail.


Inserting at the tail

In the aboveinsertToTail function, we are passing down the data parameter, and then we created a new instance of the Node class. After that, we are checking if the head is empty. If so, the head itself will be set to the new node we have just after created. Otherwise, set the tail with the head and then loop through the linked list to find the tail and update the tail’s next pointer.

Inserting at Random Position

Finally, we are going to see how to insert a new node in the linked list at a given random position. For this, we have to traverse the list until we find the desired position.

Inserting at a given random position

Now we are going to test this function using the next tests.

The output will be as below. As you can see, at the given index, the node (600) was added at the second index of the list.

The Whole Code

class Node {
  constructor(data, next = null) {
    this.data = data;
    this.next = next;
  }
}

//Let's create four nodes
let node1 = new Node(5);
let node2 = new Node(10);
let node3 = new Node(20);
let node4 = new Node(1);

//connecting nodes
node1.next = node2;
node2.next = node3;
node3.next = node4;

//LinkedList Class
class LinkedList {
  constructor() {
    this.head = null; //first node of the Linked List
    this.size = 0; //Track size of the linked list
  }
  //Insert to head
  insertToHead(data) {
    this.head = new Node(data, this.head);
    this.size++;
  }

  //Insert into the tail
  insertToTail(data) {
    const node = new Node(data);
    let tail = null;
    //if empty, make it head
    if (!this.head) {
      this.head = node;
    } else {
      tail = this.head;
      while (tail.next) {
        tail = tail.next;
      }
      tail.next = node;
    }
    this.size++;
  }

  //Insert at random position
  insertAt(data, index) {
    //if it's empty
    if (!this.head) {
      this.head = new Node(data);
      return;
    }
    //if it needs add to the front of the list
    if (index === 0) {
      this.insertToHead(data); //reuse insertToHead function
      return;
    }
    let node = new Node(data);
    let current, previous;
    let count = 0;
    // current will be first
    current = this.head;
    while (count < index) {
      previous = current;
      count++;
      current = current.next;
    }
    node.next = current;
    previous.next = node;
    this.size++;
  }
}

const linkedList = new LinkedList();
linkedList.insertToHead(100);
linkedList.insertToHead(200);
linkedList.insertToHead(300);
linkedList.insertToTail(400);
linkedList.insertAt(600, 2);

console.table(linkedList);

LinkedList.js

I hope you have gained more knowledge about data structure and especially with Linked list. That’s all for now.

Thanks for reading!

An Introduction to JavaScript ES6 Proxies

An Introduction to JavaScript ES6 Proxies

Proxy is one of the most overlooked concepts introduced in ES6 version of JavaScript, but ES6 proxies bound to come in handy at some point in your future.

Proxy is one of the most overlooked concepts introduced in the ES6 version of JavaScript.

Admittedly, it isn’t particularly useful on a day-to-day basis, but it is bound to come in handy at some point in your future.

The basics

The Proxy object is used to define a custom behavior for fundamental operations such as property lookup, assignment, and function invocation.

The most basic example of a proxy would be:

const obj = {
 a: 1,
 b: 2,
};

const proxiedObj = new Proxy(obj, {
 get: (target, propertyName) => {
   // get the value from the "original" object
   const value = target[propertyName];

   if (!value && value !== 0) {
     console.warn('Trying to get non-existing property!');

     return 0;
   }

   // return the incremented value
   return value + 1;
 },
 set: (target, key, value) => {
   // decrement each value before saving
   target[key] = value - 1;

   // return true to indicate successful operation
   return true;
 },
});

proxiedObj.a = 5;

console.log(proxiedObj.a); // -> incremented obj.a (5)
console.log(obj.a); // -> 4

console.log(proxiedObj.c); // -> 0, logs the warning (the c property doesn't exist)

We have intercepted the default behavior of both get and set operations by defining the handlers with their respective names in the object provided to the proxy constructor. Now each get operation will return the incremented value of the property, while set will decrement the value before saving it in the target object.

What’s important to remember with proxies is that once a proxy is created, it should be the only way to interact with the object.

Different kinds of traps

There are many traps (handlers that intercept the object’s default behavior) aside from get and set, but we won’t be using any of them in this article. With that being said, if you are interested in reading more about them, here’s the documentation.

Having fun

Now that we know how proxies work, let’s have some fun with them.

Observing object’s state

As it has been stated before it is very easy to intercept operations with proxies. To observe an object’s state is to be notified every time there’s an assignment operation.

const observe = (object, callback) => {
 return new Proxy(object, {
   set(target, propKey, value) {
     const oldValue = target[propKey];
   
     target[propKey] = value;

     callback({
       property: propKey,
       newValue: value,
       oldValue,
     });

     return true;
   }
 });
};

const a = observe({ b: 1 }, arg => {
 console.log(arg);
});

a.b = 5; // -> logs from the provided callback: {property: "b", oldValue: 1, newValue: 5}

And that’s all we have to do — invoke the provided callback every time the set handler is fired.

As an argument to the callback, we provide an object with three properties: the name of the changed property, the old value, and the new value.

Prior to executing the callback, we assign the new value in the target object so the assignment actually takes place. We have to return true to indicate that the operation has been successful; otherwise, it would throw a TypeError.

Here’s a live example.

Validating properties on set

If you think about it, proxies are a good place to implement validation — they are not tightly coupled with the data itself. Let’s implement a simple validation proxy.

As in the previous example, we have to intercept the set operation. We would like to end up with the following way of declaring data validation:

const personWithValidation = withValidation(person, {
 firstName: [validators.string.isString(), validators.string.longerThan(3)],
 lastName: [validators.string.isString(), validators.string.longerThan(7)],
 age: [validators.number.isNumber(), validators.number.greaterThan(0)]
});

In order to achieve this, we define the withValidation function like so:

const withValidation = (object, schema) => {
 return new Proxy(object, {
   set: (target, key, value) => {
     const validators = schema[key];

     if (!validators || !validators.length) {
       target[key] = value;

       return true;
     }

     const shouldSet = validators.every(validator => validator(value));

     if (!shouldSet) {
       // or get some custom error
       return false;
     }

     target[key] = value;
     return true;
   }
 });
};

First we check whether or not there are validators in the provided schema for the property that is currently being assigned — if there aren’t, there is nothing to validate and we simply assign the value.

If there are indeed validators defined for the property, we assert that all of them return true before assigning. Should one of the validators return false, the whole set operation returns false, causing the proxy to throw an error.

The last thing to do is to create the validators object.

const validators = {
 number: {
   greaterThan: expectedValue => {
     return value => {
       return value > expectedValue;
     };
   },
   isNumber: () => {
     return value => {
       return Number(value) === value;
     };
   }
 },
 string: {
   longerThan: expectedLength => {
     return value => {
       return value.length > expectedLength;
     };
   },
   isString: () => {
     return value => {
       return String(value) === value;
     };
   }
 }
};

The validators object contains validation functions grouped by the type they should validate. Each validator on invocation takes the necessary arguments, like validators.number.greaterThan(0), and returns a function. The validation happens in the returned function.

We could extend the validation with all kinds of amazing features, such as virtual fields or throwing errors from inside the validator to indicate what went wrong, but that would make the code less readable and is outside the scope of this article.

Here’s a live example.

Making code lazy

For the final — and hopefully most interesting — example, let’s create a proxy that makes all the operations lazy.

Here’s a very simple class called Calculator, which contains a few basic arithmetic operations.

class Calculator {
 add(a, b) {
   return a + b;
 }

 subtract(a, b) {
   return a - b;
 }

 multiply(a, b) {
   return a * b;
 }

 divide(a, b) {
   return a / b;
 }
}

Now normally, if we ran the following line:

new Calculator().add(1, 5) // -> 6

The result would be 6.

The code is executed on the spot. What we would like is to have the code wait for the signal to be run, like a run method. This way the operation will be postponed until it is needed — or not executed at all if there is never a need.

So the following code, instead of 6, would return the instance of the Calculator class itself:

lazyCalculator.add(1, 5) // -> Calculator {}

Which would give us another nice feature: method chaining.

lazyCalculator.add(1, 5).divide(10, 10).run() // -> 1

The problem with that approach is that in divide, we have no clue of what the result of add is, which makes it kind of useless. Since we control the arguments, we can easily provide a way to make the result available through a previously defined variable — $, for example.

lazyCalculator.add(5, 10).subtract($, 5).multiply($, 10).run(); // -> 100

$ here is just a constant Symbol. During execution, we dynamically replace it with the result returned from the previous method.

const $ = Symbol('RESULT_ARGUMENT');

Now that we have a fair understanding of what do we want to implement, let’s get right to it.

Let’s create a function called lazify. The function creates a proxy that intercepts the get operation.

function lazify(instance) {
 const operations = [];

 const proxy = new Proxy(instance, {
   get(target, propKey) {
     const propertyOrMethod = target[propKey];

     if (!propertyOrMethod) {
       throw new Error('No property found.');
     }

     // is not a function
     if (typeof propertyOrMethod !== 'function') {
       return target[propKey];
     }

     return (...args) => {
       operations.push(internalResult => {
         return propertyOrMethod.apply(
           target,
           [...args].map(arg => (arg === $ ? internalResult : arg))
         );
       });

       return proxy;
     };
   }
 });

 return proxy;
}

Inside the get trap, we check whether or not the requested property exists; if it doesn’t, we throw an error. If the property is not a function, we return it without doing anything.

Proxies don’t have a way of intercepting method calls. Instead, they are treating them as two operations: the get operation and a function invocation. Our get handler has to act accordingly.

Now that we are sure the property is a function, we return our own function, which acts as a wrapper. When the wrapper function is executed, it adds yet another new function to the operations array. The wrapper function has to return the proxy to make it possible to chain methods.

Inside the function provided to the operations array, we execute the method with the arguments provided to the wrapper. The function is going to be called with the result argument, allowing us to replace all the $ with the result returned from the previous method.

This way we delay the execution until requested.

Now that we have built the underlying mechanism to store the operations, we need to add a way to run the functions — the .run() method.

This is fairly easy to do. All we have to do is check whether the requested property name equals run. If it does, we return a wrapper function (since run acts as a method). Inside the wrapper, we execute all the functions from the operations array.

The final code looks like this:

const executeOperations = (operations, args) => {
 return operations.reduce((args, method) => {
   return [method(...args)];
 }, args);
};

const $ = Symbol('RESULT_ARGUMENT');

function lazify(instance) {
 const operations = [];

 const proxy = new Proxy(instance, {
   get(target, propKey) {
     const propertyOrMethod = target[propKey];

     if (propKey === 'run') {
       return (...args) => {
         return executeOperations(operations, args)[0];
       };
     }

     if (!propertyOrMethod) {
       throw new Error('No property found.');
     }

     // is not a function
     if (typeof propertyOrMethod !== 'function') {
       return target[propKey];
     }

     return (...args) => {
       operations.push(internalResult => {
         return propertyOrMethod.apply(
           target,
           [...args].map(arg => (arg === $ ? internalResult : arg))
         );
       });

       return proxy;
     };
   }
 });

 return proxy;
}

The executeOperations function takes an array of functions and executes them one by one, passing the result of the previous one to the invocation of the next one.

And now for the final example:

const lazyCalculator = lazify(new Calculator());

const a = lazyCalculator
 .add(5, 10)
 .subtract($, 5)
 .multiply($, 10);

console.log(a.run()); // -> 100

If you are interested in adding more functionality I have added a few more features to the lazify function — asynchronous execution, custom method names, and a possibility to add custom functions through the .chain() method. Both versions of the lazify function are available in the live example.

Summary

Now that you have seen proxies in action, I hope that you could find a good use for them in your own codebase.

Proxies have many more interesting uses than those covered here, such as implementing negative indices and catching all the nonexistent properties in an object. Be careful, though: proxies are a bad choice when performance is an important factor.