Javascript

Types and Grammar

Contents

• Types
• Values
• Natives
• Coercion
• Grammar

Types (again!?)

• null
• undefined
• boolean
• number
• string
• object
• symbol (ES6)

typeof foo(){...} == "function"

BUT

functions are callable objects

Values as Types

• vales has types
• variables don't have types
• no static typing

var a = 5;
typeof a; // "number"

a = "different value";
typeof a // "string"

undefined vs undeclared

var x;

x; // undefined
y; // ReferenceError: y is not defined

WTF: type of undeclared variable is undefined

var x;

typeof x; // "undefined"
typeof y; // "undefined"

Values

• Arrays
  • store any types
  • object with additional prototype methods
• Array-likes
  • List of DOM elements
  • Cannot use Array.prototype methods
• string
  • like Array of character but not every Array methods work
  • immutable
• number
  • no distinction between integer and float

Numbers

• NaN
• Infinity / -Infinity
• -0 / 0

value vs reference

• number, string, boolean are copied by value (immutable)
• Arrays, objects are copied by reference (mutable)

Natives

• String( )
• Number()
• Boolean()
• Array()
• Object()
• Function()
• RegExp()
• Date()
• Error()
• Symbol()

Boxing

Primitives can use methods of natives

Unboxing

Getting underlying primitive from natives

var a = 'abc'

a.length; // 3
a.toUpperCase(); // "ABC"

var a = new String('abc');

a; // [String: 'abc']
a.valueOf(); // "abc"

Avoid natives if there's no reason to use

• {} instead of new Object()
• function foo () {...} instead of new Function("foo", "...")
• [] instead of new Array()

Coercion

• Implicit type conversion 
• type casting: explicit type conversion
• Coercion is ended with primitives 
• toString, toBoolean, toNumber
• loose equality (==) uses coercion

Grammar

• {..} => object or block scope
• if / else if / else
• Operator precedence
• Automatic semicolon insertion (ASI)
• try, catch, finally
• switch

Keep coding Javascript...