Type Conversions

運算符號和函式會自動把值轉換成正確的類型，像是 alert 把值轉換成 string，運算符號把值轉換成數字、有些情況我們需要把值轉換成需要的類型。

ToString

let value = true;
alert(typeof value); // boolean

value = String(value); // now value is a string "true"
alert(typeof value); // string

ToNumber

運算是跟函式都可以轉換成數字。

alert( "6" / "2" ); // 3, operator strings are converted to numbers

let str = "123";
alert(typeof str); // string

let num = Number(str); // becomes a number 123

alert(typeof num); // number

如果轉換的值不是數字，會顯示 NaN。

let age = Number("an arbitrary string instead of a number");

alert(age); // NaN, conversion failed

轉換數字的規則：

Value	Becomes…
undefined	NaN
null	0
true and false	1 and 0
string	Whitespaces from the start and end are removed. If the remaining string is empty, the result is 0. Otherwise, the number is “read” from the string. An error gives NaN.

+ 運算符號是例外，如果其中一個值是 string 會變成字串相加，而不是轉換成數字。

alert( 1 + '2' ); // '12' (string to the right)
alert( '1' + 2 ); // '12' (string to the left)

ToBoolean

轉換布林值規則：

Value	Becomes…
0, null, undefined, NaN, ""	false
any other value	true

alert( Boolean("0") ); // true
alert( Boolean(" ") ); // spaces, also true (any non-empty string is true)