Deep-Javascript-Foundation-V3.md

Primitive types

undefined string number boolean object symbol

null bigint (future)

undeclared

Objects

object function callable object array subtype

Operator typeof var v; typeof v; 'undefined'

v = "1"; typeof v; 'string'

v = 2; typeof v; 'number'

v = true; typeof v; 'boolean'

v = {}; typeof v; 'object'

v = Symbol(); typeof v; 'symbol'

typeof doesNotExists 'undefined'

var v = null; typeof v; 'object'

v = function() {}; typeof v; 'function'

v = []; // v = Array(); // v = new Array(); typeof v; 'object' Array.isArray() method determines whether the passed value is an Array.

var v = 44n; typeof v; 'bigint'

undefined vs undeclared

uninitialized Temporal Dead Zone (TDZ) Refers to the region before a variable (const let) is defined.

console.log(test1); // undefined var test1 = 'foo';

console.log(test2); // ReferenceError: cannot access uninitialized variable const test2 = 'foo';

console.log(test3); // ReferenceError: cannot access uninitialized variable let test3 = 'foo';

Special Values

NaN -> invalid number

var myAge = Number("0o46") // 38

var myNextAge = Number("39"); // 39

var myCatAge = Number("n/a") // NaN

myAge - "My son age"; // NaN

Nan not identity value myCatAge === myCatAg // false

isNaN(myAge) // false

isNaN(myCatAge) // true

isNaN("My sons age") // true OOPS!

reason because its Coercion to number

Preferred way use Number Rescue Number.isNaN(myCatAge); // true Number.isNaN("My sons age") // as expected false

Negative Zero

var trendRate = -0;

trendRate === -0 // true

trendRate.toString() // "0" OOPS! trendRate === 0; // true OOPS!

trendRate < 0 // false HOW? trendRate > 0 // false

Rescue Object.is(trendRate, -0) // true Object.is(trendRate, 0) // false

Fundamental Objects

Built in Objects Native Functions Built new Number() or new String()

Use new:

• Object()
• Array()
• Function()
• Date()
• RegExp()
• Error()

Don't use with new

• String()
• Number()
• Boolean()

var yesterday = new ("March 6, 2019");

yesterday.toUTCString(); // "Wed, 06 Mar 2019 06:00:00 GMT"

var myGPA = String(transcript.gap);

Abstract Operations

Type Conversion

toString()

null => "null" undefined => "undefined" true => "true" false => "false" 3.14159 => "3.14159" 0 => "0" -0 => "-0"

[] Array to string returns "" [] undefined,null value get remove and multiple values get , separated ["",false, undefined, null,,,,,0,-0,,-1,NaN].toString(); // ',false,,,,,,,0,0,,-1,NaN' Value of on empty ["",false, undefined, null,,,,,0,-0,,-1,NaN].valueOf() // ['', false, undefined, null, empty × 4, 0, -0, empty, -1, Na

{}.toString() [object,Object]

ToNumber "" => 0 "0" => 0 "-0" => -0 " 0009 " => 9 // remove white space and leading zero "0." => 0 "." => NaN "0xaf" => 175

false 0 true 1 null 0 undefined NaN

To Boolean is table lookup of falsy value "" 0, -0 null NaN false undefined