JavaScript Basics

Data Types

• Primitive data types

  • number // var a = 5;
  • boolean // var c = true;
  • string // var b = “hello”;
  • null // var d = null
  • undefined // var e = undefined

• Data Types

  • Object

Numbers

Predefined values

• Infinity
• NaN
• Number.MAX_VALUE
• Number.MIN_VALUE
• Number.NaN
• Number.POSITIVE_INFINITY
• Number.NEGATIVE_INFINITY

NaN

1. Result of erroneous operations (for example "string" / 2)
2. Any arithmetic operation with NaN as an input will have NaN as a result
3. NaN is not equal to anything, including NaN
4. Use IsNaN() to determine whether a value is NaN

   var number = "string" / 2;
   if (!isNaN(number)) {
     // todo something
   }

Boolean

A JavaScript Boolean represents one of two values: true or false.

Watch out

False	True
false	"false"
null	"0"
undefined	Boolean("false");
""
0
Number.NaN

Null and Undefined

	string	number	boolean
undefined	"undefined"	NaN	false
null	"null"	0	false

Object

Objects can be created using the Object() constructor or the object initializer / literal syntax (const object1 = { a: 'foo', b: 42, c: {} };).

Object.prototype has following methods:

• constructor
• toString()
• valueOf()
• hasOwnProperty(“name”)
• propertyIsEnumerable(“name”)
• isPrototypeOf(“object”)

Monkey patching: Object.prototype.x = 10;

Array

• Array inherits from Object
• Indexes are converted to strings and used as names for retrieving values
• Arrays have a special length member (array.length)

var array = new Array(1, 2, 3);
var array = [1, 2, 3];
var array = [
  1,
  "234234",
  true,
  function () {
    alert("hello!");
  },
];
array.length;

// Removes the element, but leaves a hole in the numbering.
delete array[number];

Handy Array methods:

• var arrayAsString = array.join("separator");
• array.reverse();
• array.sort(/_ options: comparison function _/);//important
• var newArray = array.concat("array");
• var subarray = array.slice(“startIndex”,”lastIndex”);
• array.splice(“startIndex”,”itemsToRemove”,/new items/);
• var newArrayLength = array.push(“value”);
• var removedValue= array.pop();
• var newArrayLength = array.unshift(“value”) ;
• var removedValue= array.shift();

Operators

"typeof" operator

To check if variable is defined typeof foo !== ‘undefined’

typeof	Result
undefined	"undefined"
null	"object"
true	"boolean"
new Boolean(true)	"object"
5	"number"
new Number(5)	"object"
“foo”	"string"
new String(“foo”)	"object"
[1, 2, 3]	"object"
function foo()	"function"
Any other object	"object"

"+" operator

if both (lef and right) are numbers
then
  add them as numbers
else
  convert both to string
  concatenate them

For example:

"$" + 3 + 4; //'$34'
"$" + (3 + 4); //'$7'

Equal "==" and and not equal "!=" operators

• These operators can do type coercion
• It is always better to use === and !==, which do not do type coercion.

"" == "0"; // false
0 == ""; // true
0 == "0"; // true
false == "false"; // false
false == "0"; // true
false == undefined; // false
false == null; // false
null == undefined; // true
"\t\r\n" == 0; // true

JavaScript Scopes

JavaScript has following scopes.

Block Scope

Was introduced with ES6 (2015) along with important new JavaScript keywords (let and const) to ensure that variables, which declared inside a {} block are not accesable from outside the block.

{
  let localVar = 2;
}
// localVar is undefined

Function Scope

Variables declared within a JavaScript function has funcition scope.

function Foo() {
  var localVar = "local variable with function scope";
}

Global Scope

A variable declared outside a function, becomes GLOBAL.

let carName = "Volvo";
// code here can use carName

function myFunction() {
// code here can also use carName
}