September 24, 2014

Javascript - 01 - Variables, Data Type

Javascript has different version because of the different standard.

JavaScript version	Navigator version
JavaScript 1.0	Navigator 2.0
JavaScript 1.1	Navigator 3.0
JavaScript 1.2	Navigator 4.0-4.05
JavaScript 1.3	Navigator 4.06-4.7x
JavaScript 1.4
JavaScript 1.5	Navigator 6.0 Mozilla (open source browser)
JavaScript 1.6	Firefox 1.5, other Mozilla 1.8-based products
JavaScript 1.7	Firefox 2, other Mozilla 1.8.1-based products
JavaScript 1.8	Firefox 3, other Gecko 1.9-based products

Data types

Type	Examples of typed values / Notes
Numbers	42, 3.14159
Logical (Boolean)	true / false
Strings	"Howdy"
null	a special keyword denoting a null value; `null` is also a primitive value. Because JavaScript is case-sensitive, `null` is not the same as `Null`, `NULL`, or any other variant
undefined	a top-level property whose value is undefined; `undefined` is also a primitive value.

Define variables

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Values,_variables,_and_literals

var x = 10; //local variables.
y = 10; //global variables.

Variable default value is undefined.

NaN is a property of the global object.

NaN === NaN; // false
Number.NaN === NaN; // false
isNaN(NaN); // true
isNaN(Number.NaN); // true

The initial value of NaN is Not-A-Number — the same as the value of Number.NaN. In modern browsers, NaN is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it.

Constant

const prefix = '212';

Array

var fish = ["Lion", , "Angel"];

Object Literals

var Sales = "Toyota";
function CarTypes(name) {
 if (name == "Honda") {
 return name;
 } else {
 return "Sorry, we don't sell " + name + ".";
 }
}
var car = { myCar: "Saturn", getCar: CarTypes("Honda"), special: Sales };
console.log(car.myCar); // Saturn
console.log(car.getCar); // Honda
console.log(car.special); // Toyota
/*=================================Nested Object=========================*/
var car = { manyCars: {a: "Saab", "b": "Jeep"}, 7: "Mazda" };
console.log(car.manyCars.b); // Jeep
console.log(car[7]); // Mazda
/*===========================Empty String Identifier===================*/
var unusualPropertyNames = {
 "": "An empty string",
 "!": "Bang!"
}
console.log(unusualPropertyNames.""); // SyntaxError: Unexpected string
console.log(unusualPropertyNames[""]); // "An empty string"
console.log(unusualPropertyNames.!); // SyntaxError: Unexpected token !
console.log(unusualPropertyNames["!"]); // "Bang!"
//The empty string Identifier is able to compiled , but not as used.
/*=========================Unusual Case on Selector==================*/
var foo = {a: "alpha", 2: "two"};
console.log(foo.a); // alpha
console.log(foo[2]); // two
//console.log(foo.2); // Error: missing ) after argument list
//console.log(foo[a]); // Error: a is not defined
console.log(foo["a"]); // alpha
console.log(foo["2"]); // two

Special Characters

Character	Meaning
`\b`	Backspace
`\f`	Form feed
`\n`	New line
`\r`	Carriage return
`\t`	Tab
`\v`	Vertical tab
`\'`	Apostrophe or single quote
`\"`	Double quote
`\\`	Backslash character
`\XXX`	The character with the Latin-1 encoding specified by up to three octal digits XXX between 0 and 377. For example, \251 is the octal sequence for the copyright symbol.
`\xXX`	The character with the Latin-1 encoding specified by the two hexadecimal digits XX between 00 and FF. For example, \xA9 is the hexadecimal sequence for the copyright symbol.
`\uXXXX`	The Unicode character specified by the four hexadecimal digits XXXX. For example, \u00A9 is the Unicode sequence for the copyright symbol. SeeUnicode escape sequences.