Array

---

General

Info:

• arrays are written with square brackets.
• Array items are separated by commas.
• Arrays are special type of objects
  • Elements can be arrays, functions and objects
• Arrays use numbers to access its "elements".
  • Array indexes start with 0
  • The length property is always one more than the highest array index.

Caution:

• Comparing string properties are more complex
• The reduce() method does not reduce the original array
• The typeof operator in JavaScript returns "object" for arrays.

Danger:

• Adding elements with high indexes can create undefined "holes" in an array.
• Using delete leaves undefined holes in the array.
• Arrays use numbered indexes.
  • If you use named indexes, JavaScript will redefine the array to an object. After that, some array methods and properties will produce incorrect results.

const person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
person.length;     // Will return 0
person[0];         // Will return undefined

• JavaScript has a built-in array constructor new Array().

  • Avoid using Array constructor

  • The new keyword can produce some unexpected results:

    // Create an array with one element ???
    const points = new Array(40);  
    ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

• sort() method will produce incorrect result when sorting numbers:

  "25" is bigger than "100"

Declaration

---

const

Caution:

• Arrays are Not Constants. It defines a constant reference to an array.

Danger:

• JavaScript const variables must be assigned a value when they are declared:

  const cars;
  cars = ["Saab", "Volvo", "BMW"];
• An array declared with const cannot be reassigned

  const cars = ["Saab", "Volvo", "BMW"];
  cars = ["Toyota", "Volvo", "Audi"];

var

• Redeclaring an array declared with var is allowed anywhere

  var cars = ["Volvo", "BMW"];   // Allowed
  var cars = ["Toyota", "BMW"];  // Allowed
  cars = ["Volvo", "Saab"];      // Allowed

• Arrays declared with var can be initialized at any time.

  • You can even use the array before it is declared:

    cars = ["Saab", "Volvo", "BMW"];
    var cars;

Scope

---

const

Info:

• const has Block Scope.

Danger:

• An array declared in a block is not the same as an array declared outside the block:

  const cars = ["Saab", "Volvo", "BMW"];
  // Here cars[0] is "Saab"
  {
    const cars = ["Toyota", "Volvo", "BMW"];
    // Here cars[0] is "Toyota"
  }
  // Here cars[0] is "Saab"

• Redeclaring or reassigning an array to const, in the same scope, or in the same block, is not allowed:

  const cars = ["Volvo", "BMW"];
  const cars = ["Volvo", "BMW"];
  var cars = ["Volvo", "BMW"];
  cars = ["Volvo", "BMW"]; 
  
  {
    const cars = ["Volvo", "BMW"];
    const cars = ["Volvo", "BMW"];
    var cars = ["Volvo", "BMW"];
    cars = ["Volvo", "BMW"]; 
  }

var

Info:

• var has global Scope

  var cars = ["Saab", "Volvo", "BMW"];
  // Here cars[0] is "Saab"
  {
    var cars = ["Toyota", "Volvo", "BMW"];
    // Here cars[0] is "Toyota"
  }
  // Here cars[0] is "Toyota"