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Learn JavaScript Syntax: Objects

JavaScript destructuring assignment shorthand syntax

The JavaScript destructuring assignment is a shorthand syntax that allows object properties to be extracted into specific variable values.

It uses a pair of curly braces ({}) with property names on the left-hand side of an assignment to extract values from objects. The number of variables can be less than the total properties of an object. 

const rubiksCubeFacts = {
  possiblePermutations: '43,252,003,274,489,856,000',
  invented: '1974',
  largestCube: '17x17x17'
};
const {possiblePermutations, invented, largestCube} = rubiksCubeFacts;
console.log(possiblePermutations); // '43,252,003,274,489,856,000'
console.log(invented); // '1974'
console.log(largestCube); // '17x17x17'

shorthand property name syntax for object creation

The shorthand property name syntax in JavaScript allows creating objects without explicitly specifying the property names (ie. explicitly declaring the value after the key). In this process, an object is created where the property names of that object match variables which already exist in that context. Shorthand property names populate an object with a key matching the identifier and a value matching the identifier’s value.

const activity = 'Surfing';
const beach = { activity };
console.log(beach); // { activity: 'Surfing' }

this Keyword

The reserved keyword this refers to a method’s calling object, and it can be used to access properties belonging to that object.

Here, using the this keyword inside the object function to refer to the cat object and access its name property.

const cat = {
  name: 'Pipey',
  age: 8,
  whatName() {
    return this.name  
  }
};


console.log(cat.whatName()); 
// Output: Pipey

javascript function this

Every JavaScript function or method has a this context. For a function defined inside of an object, this will refer to that object itself. For a function defined outside of an object, this will refer to the global object (window in a browser, global in Node.js). 

const restaurant = {
  numCustomers: 45,
  seatCapacity: 100,
  availableSeats() {
    // this refers to the restaurant object
    // and it's used to access its properties
    return this.seatCapacity - this.numCustomers;
  }
}

JavaScript Arrow Function this Scope

JavaScript arrow functions do not have their own this context, but use the this of the surrounding lexical context. Thus, they are generally a poor choice for writing object methods.

Consider the example code:

loggerA is a property that uses arrow notation to define the function. Since data does not exist in the global context, accessing this.data returns undefined.

loggerB uses method syntax. Since this refers to the enclosing object, the value of the data property is accessed as expected, returning "abc".

const myObj = {
    data: 'abc',
    loggerA: () => { console.log(this.data); },
    loggerB() { console.log(this.data); },
};


myObj.loggerA();    // undefined
myObj.loggerB();    // 'abc'

getters and setters intercept property access

JavaScript getter and setter methods are helpful in part because they offer a way to intercept property access and assignment, and allow for additional actions to be performed before these changes go into effect. 

const myCat = {
  _name: 'Snickers',
  get name(){
    return this._name
  },
  set name(newName){
    //Verify that newName is a non-empty string before setting as name property
    if (typeof newName === 'string' && newName.length > 0){
      this._name = newName; 
    } else {
      console.log("ERROR: name must be a non-empty string"); 
    }
  }
}

javascript factory functions

A JavaScript function that returns an object is known as a factory function. Factory functions often accept parameters in order to customize the returned object. 

// A factory function that accepts 'name', 
// 'age', and 'breed' parameters to return 
// a customized dog object. 
const dogFactory = (name, age, breed) => {
  return {
    name: name,
    age: age,
    breed: breed,
    bark() {
      console.log('Woof!');  
    }
  };
};

javascript getters and setters restricted

JavaScript object properties are not private or protected. Since JavaScript objects are passed by reference, there is no way to fully prevent incorrect interactions with object properties.

One way to implement more restricted interactions with object properties is to use getter and setter methods.

Typically, the internal value is stored as a property with an identifier that matches the getter and setter method names, but begins with an underscore (_).

const myCat = {
  _name: 'Dottie',
  get name() {
    return this._name;  
  },
  set name(newName) {
    this._name = newName;  
  }
};


// Reference invokes the getter
console.log(myCat.name);


// Assignment invokes the setter
myCat.name = 'Yankee';

Restrictions in Naming Properties

JavaScript object key names must adhere to some restrictions to be valid. Key names must either be strings or valid identifier or variable names (i.e. special characters such as - are not allowed in key names that are not strings).

// Example of invalid key names
const trainSchedule = {
  platform num: 10, // Invalid because of the space between words.
  40 - 10 + 2: 30, // Expressions cannot be keys.
  +compartment: 'C' // The use of a + sign is invalid unless it is enclosed in quotations.
}

Dot Notation for Accessing Object Properties

Properties of a JavaScript object can be accessed using the dot notation in this manner: object.propertyName. Nested properties of an object can be accessed by chaining key names in the correct order.

const apple = { 
  color: 'Green',
  price: {
    bulk: '$3/kg',
    smallQty: '$4/kg'
  }
};
console.log(apple.color); // 'Green'
console.log(apple.price.bulk); // '$3/kg'

Objects

An object is a built-in data type for storing key-value pairs. Data inside objects are unordered, and the values can be of any type.

Accessing non-existent JavaScript properties

When trying to access a JavaScript object property that has not been defined yet, the value of undefined will be returned by default.

const classElection = {
  date: 'January 12'
};


console.log(classElection.place); // undefined

JavaScript Objects are Mutable

JavaScript objects are mutable, meaning their contents can be changed, even when they are declared as const. New properties can be added, and existing property values can be changed or deleted.

It is the reference to the object, bound to the variable, that cannot be changed.

const student = {
  name: 'Sheldon',
  score: 100,
  grade: 'A',
}


console.log(student)
// { name: 'Sheldon', score: 100, grade: 'A' }


delete student.score
student.grade = 'F'
console.log(student)
// { name: 'Sheldon', grade: 'F' }


student = {}
// TypeError: Assignment to constant variable.

JavaScript for...in loop

The JavaScript for...in loop can be used to iterate over the keys of an object. In each iteration, one of the properties from the object is assigned to the variable of that loop.

let mobile = {
  brand: 'Samsung',
  model: 'Galaxy Note 9'
};


for (let key in mobile) {
  console.log(`${key}: ${mobile[key]}`);
}

Properties and values of a JavaScript object

A JavaScript object literal is enclosed with curly braces {}. Values are mapped to keys in the object with a colon (:), and the key-value pairs are separated by commas. All the keys are unique, but values are not.

Key-value pairs of an object are also referred to as properties.

const classOf2018 = {
  students: 38,
  year: 2018
}

Delete operator

Once an object is created in JavaScript, it is possible to remove properties from the object using the delete operator. The delete keyword deletes both the value of the property and the property itself from the object. The delete operator only works on properties, not on variables or functions. 

const person = {
  firstName: "Matilda",
  age: 27,
  hobby: "knitting",
  goal: "learning JavaScript"
};


delete person.hobby; // or delete person[hobby];


console.log(person);
/*
{
  firstName: "Matilda"
  age: 27
  goal: "learning JavaScript"
}
*/

javascript passing objects as arguments

When JavaScript objects are passed as arguments to functions or methods, they are passed by reference, not by value. This means that the object itself (not a copy) is accessible and mutable (can be changed) inside that function. 

const origNum = 8;
const origObj = {color: 'blue'};


const changeItUp = (num, obj) => {
  num = 7;
  obj.color = 'red';
};


changeItUp(origNum, origObj);


// Will output 8 since integers are passed by value.
console.log(origNum);


// Will output 'red' since objects are passed 
// by reference and are therefore mutable.
console.log(origObj.color);

JavaScript Object Methods

JavaScript objects may have property values that are functions. These are referred to as object methods.

Methods may be defined using anonymous arrow function expressions, or with shorthand method syntax.

Object methods are invoked with the syntax: objectName.methodName(arguments).

const engine = {
  // method shorthand, with one argument
  start(adverb) {
    console.log(`The engine starts up ${adverb}...`);
  },  
  // anonymous arrow function expression with no arguments
  sputter: () => {
    console.log('The engine sputters...');
  },
};


engine.start('noisily');
engine.sputter();


/* Console output:
The engine starts up noisily...
The engine sputters...
*/
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