摘要：原文測試對象包裝基礎數據類型沒有屬性和方法，為了使用方法和函數，就需要對應的對象包裝它。注意，用構造器構造的對象，永遠是。它們都是對象。它們都是非空字符串。

You Don"t Know JS: Types & Grammar

console.log(1+ "2"+"2");
console.log(1+ +"2"+"2");
console.log("A"- "B"+"2");
console.log("A"- "B"+2);

122,32,NaN2,NaN

String()

Number()

Boolean()

Array()

Object()

Function()

RegExp()

Date()

Error()

Symbol()

var a = new String( "abc" );

typeof a; // "object" ... not "String"

a instanceof String; // true

Object.prototype.toString.call( a ); // "[object String]"
console.log(a); // String {[[PrimitiveValue]]: "abc"}

基礎數據類型沒有屬性和方法，為了使用方法和函數，就需要對應的對象包裝它。JS可以自動做到這一點。

var a = "abc";

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

注意，用構造器構造的對象，永遠是“truthy”。

var a = new Boolean( false );

if (!a) {
    console.log( "Oops" ); // never runs
}

默認調用toString()

[1,3,4]+"haha" // "1,3,4haha"
"haha" + {f:"da"} // "haha[object Object]"

{f:"da"}+"haha" // NaN
var f = {f:"da"}
f+"haha" // "[object Object]haha"

數組默認調用toString()

JSON字符串化，會忽略undefined,function,symbol，循環引用的對象還會報錯。

JSON.stringify( 42 );    // "42"
JSON.stringify( "42" );    // ""42"" (a string with a quoted string value in it)
JSON.stringify( null );    // "null"
JSON.stringify( true );    // "true"

JSON.stringify( undefined );                    // undefined
JSON.stringify( function(){} );                    // undefined

JSON.stringify( [1,undefined,function(){},4] );    // "[1,null,null,4]"
JSON.stringify( { a:2, b:function(){} } );        // "{"a":2}"

JSON.stringify(obj, replacer, space)

true變成1，false變成0，undefined變成NaN，null變成0，""變成0，[]變成0

當進行數值計算時，數字型字符串變成數字，非數字型字符串變成NaN，對象先通過toSting()變成字符串，再參與計算。

[2]*2 // 4
[2,3]*2 // NaN
["2"]*2 // 4
"A"*2 // NaN
null+2 // 2
""*2 // 0

falsy values

undefined

null

false

+0,-0,NaN

""

!!"" // false
!!"  " // true
""*1 // 0
"  "*1 // 0

var a = new Boolean( false );
var b = new Number( 0 );
var c = new String( "" );

var d = Boolean( a && b && c );

d; // true

var a = [];                // empty array -- truthy or falsy?
var b = {};                // empty object -- truthy or falsy?
var c = function(){};    // empty function -- truthy or falsy?

var d = Boolean( a && b && c );

d; // true

它們都是對象。

var a = "false";
var b = "0";
var c = """";

var d = Boolean( a && b && c );

d; // true

它們都是非空字符串。

var a = "false";
var b = "0";
var c = "";

var d = Boolean( a && b && c );

d; // false

var d = new Date( "Mon, 18 Aug 2014 08:53:06 CDT" );

+d; // 1408369986000

// or
var timestamp = +new Date();
var timestamp = new Date().getTime();
var timestamp = Date.now();

new Date().getDate() // 10
Date.now().getDay() // TypeError

var a = "42";
var b = "42px";

Number( a );    // 42
parseInt( a );    // 42

Number( b );    // NaN
parseInt( b );    // 42
parseInt("px42px"); // NaN

當傳入的非字符串先轉化為字符串，調用toString()

parseInt([14,2,3]) // 14
parseInt( new String( "42") ); // 42
var a = {
    num: 21,
    toString: function() { return String( this.num * 2 ); }
};

parseInt( a ); // 42

var a = {
    valueOf: function() { return 42; },
    toString: function() { return 4; }
};

a + "";            // "42"

String( a );    // "4"

[] + {} // "[object Object]"
{} + [] // 0

In the ES5 spec, clauses 11.9.3.4-5 say:

If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.

"0" == null;            // false
"0" == undefined;        // false
"0" == false;            // true -- UH OH!
"0" == NaN;                // false
"0" == 0;                // true
"0" == "";                // false

false == null;            // false
false == undefined;        // false
false == NaN;            // false
false == 0;                // true -- UH OH!
false == "";            // true -- UH OH!
false == [];            // true -- UH OH!
false == {};            // false

"" == null;                // false
"" == undefined;        // false
"" == NaN;                // false
"" == 0;                // true -- UH OH!
"" == [];                // true -- UH OH!
"" == {};                // false

0 == null;                // false
0 == undefined;            // false
0 == NaN;                // false
0 == [];                // true -- UH OH!
0 == {};                // false

瘋狂的例子

[] == ![];        // true
2 == [2];        // true
"" == [null];    // true
0 == "
";        // true

來源:https://github.com/dorey/JavaScript-Equality-Table

var a = [ 42 ];
var b = [ "43" ];

a < b;    // true
b < a;    // false

var a = [ "42" ];
var b = [ "043" ];

a < b;    // false

var a = [ 4, 2 ];
var b = [ 0, 4, 3 ];

a < b;    // false

var a = { b: 42 };
var b = { b: 43 };

a < b;    // false
a == b;    // false
a > b;    // false

a <= b;    // true
a >= b;    // true

Since b < a is also false, the result of a <= b is true.

var a = [ 42 ];
var b = "043";

a < b;                        // false -- string comparison!
Number( a ) < Number( b );    // true -- number comparison!

有必要改天重新整理下