JavaScript: The Definitive Guide, 6th Edition

Type Conversions

JavaScript is very flexible about the types of values it requires. We’ve seen this for booleans: when JavaScript expects a boolean value, you may supply a value of any type, and JavaScript will convert it as needed. Some values (“truthy” values) convert to true and others (“falsy” values) convert to false. The same is true for other types: if JavaScript wants a string, it will convert whatever value you give it to a string. If JavaScript wants a number, it will try to convert the value you give it to a number (or to NaN if it cannot perform a meaningful conversion). Some examples:

10 + " objects"     // => "10 objects".  Number 10 converts to a string
"7" * "4"           // => 28: both strings convert to numbers
var n = 1 - "x";    // => NaN: string "x" can't convert to a number
n + " objects"      // => "NaN objects": NaN converts to string "NaN"

Table 3-2 summarizes how values convert from one type to another in JavaScript. Bold entries in the table highlight conversions that you may find surprising. Empty cells indicate that no conversion is necessary and none is performed.

Table 3-2. JavaScript type conversions

Value	Converted to:
String	Number	Boolean	Object
undefined	"undefined"	NaN	false	throws TypeError
null	"null"	0	false	throws TypeError
true	"true"	1	new Boolean(true)
false	"false"	0	new Boolean(false)
"" (empty string)	0	false	new String("")
"1.2" (nonempty, numeric)	1.2	true	new String("1.2")
"one" (nonempty, non-numeric)	NaN	true	new String("one") ...