Objects

let user = new Object(); // "object constructor" syntax
let user = {};  // "object literal" syntax

Literals and properties

• Example:

  let user = {     // an object
  	name: "John",  // by key "name" store value "John"
  	age: 30        // by key "age" store value 30
  };
  

• Property values are accessible using the dot notation:

  // get property values of the object:
  alert( user.name ); // John
  alert( user.age ); // 30
  

• The value can be of any type (including boolean).
• To remove a property:

  delete user.age;
  

• We can also use multiword property names, but then they must be quoted.
• Rule of thumb: end the last property in the list with a comma to make it easier to add/remove/move around properties.

Square brackets

• For multiword properties, the dot access doesn’t work.
• Instead we use the square bracket notation. For example:

  let user = {
  	name: "John",
  	age: 30,
  	"likes birds": true
  };
  
  //Now, to alert
  alert(user["likes birds"]); // true
  

Property value shorthand

• We can write this:

  function makeUser(name, age) {
    return {
      name: name,
      age: age,
      // ...other properties
    };
  }
  
  let user = makeUser("John", 30);
  alert(user.name); // John
  

• For shorthand, we can use name instead of name:name, like this:

  function makeUser(name, age) {
    return {
      name, // same as name: name
      age,  // same as age: age
      // ...
    };
  }
  

• We can use both normal properties and shorthands in the same object.

Property names limitations

• A variable cannot have a name equal to one of the language-reserved words. But for an object property, there’s no such restriction. They can be any strings or symbols.
• Other types are automatically converted to strings. For instance, a number 0 becomes a string "0" when used as a property key.

Property existence test, “in” operator

• In JavaScript, it is possible to access any property in an object. There will be no error if the property doesn’t exist!
• Reading a non-existing property just returns undefined.
• There’s also a special operator "in" for that.

  let user = { name: "John", age: 30 };
  
  alert( "age" in user ); // true, user.age exists
  alert( "blabla" in user ); // false, user.blabla doesn't exist
  

The "for..in" loop

• Syntax:

  for (key in object) {
  	// executes the body for each key among object properties
  }
  

Order of an Object

• Integer properties are sorted, others appear in creation order. The details follow.
• For example:

  let codes = {
    "41": "Switzerland",
    "49": "Germany",
    "1": "USA",
  };
  
  for (let code in codes) {
    alert(code); // 1, 41, 49
  }
  

• If the keys are non-integer, then they are listed in the creation order.

  let user = {
    name: "John",
    surname: "Smith"
  };
  user.age = 25; // add one more
  
  // non-integer properties are listed in the creation order
  for (let prop in user) {
    alert( prop ); // name, surname, age
  }
  

• To fix the issue with the integer sort, we can “cheat” by making the codes non-integer. Adding a plus "+" sign before each key is enough(ex: +49).