JavaScript/Notes/TypeConversion

There are five primitive types in JavaScript: Null, Undefined, Boolean, String, Number.

Various operations in JavaScript require conversion to and from primitive values.

Converting to Boolean

When evaluating any expression that requires a boolean value, the expression must be converted into a boolean using the internal [[ToBoolean]].

For example: <source lang="javascript"> var n = 0; if(n) { // false }

var t = !""; // true. Empty string is falsy. var f = !"f"; // false. Non-empty strings are not falsy. </source>

All numbers boolean-convert to true except for the following: +/-0 and NaN

Boolean operators use type-conversion for the evaluation of their left hand side operands. <source lang="javascript"> 1 && 0; // 0. "" || 0; // 0. null || undefined; // undefined. undefined || 1; // 1. NaN || 0; // 0; </source>

All falsy values: <source lang="javascript"> false "" null undefined 0 NaN </source>

All other primitive values and all objects are truthy.

Converting to String

With the + operator, when either operand is a string, concatenation is performed.

All native objects have a toString method. Number.prototype.toString(base) is special in that it takes a base parameter.

<source lang="javascript"> 15..toString(16) </source>

Object to Primitive

Whenever the + operator is used, the operands must be converted into primitive values. First, the interpreter calls the object's valueOf to get a primitive value. If the result is a primitive value, then that value is used. Example:

<source lang="javascript"> var o = {

 valueOf : function() { return 1; } 

}; o + 1; // 2. </source>

Otherwise, if o.valueOf results in an object —and Object.prototype.valueOf does — the object's toString is called. <source lang="javascript"> var o = { toString : function() { return "1"; } }; o + 1; // "11". </source>

Converting to Number

Converting strings is a very common requirement and many approaches can be used. Any mathematical operator except the concatenation/addition operator will force type-conversion to number.

parseInt(s, radix)

To force use of a particular base, use the radix parameter: parseInt("09", base) (from 2 to 36).

If radix is omitted, the base is determined by the contents of the string. Any string beginning with 0x or 0X represents a hexadecimal number. A string beginning with a leading 0 may, in older implementations, be parsed as octal (as if raxix were 8), in ECMA-262 Ed 3 (octal digits are 0-7). If string 09 is converted to 0.

Primitive to Object

Property access operation on string, number, and boolean primitives results in the creation of a temporary object. <source lang="javascript"> // Primitive to Object conversion. true.toString(); // Boolean Object. 1.2.valueOf(); // Number object. " foo ".trim(); // String Object.

// null.toString(); // TypeError // undefined.toString(); // TypeError </source>

Type Checking

The typeof Operator

<source lang="javascript"> typeof someval; </source>

Type	Result
Undefined	"undefined"
Null	"object"
Boolean	"boolean"
Number	"number"
String	"string"
Object (native and doesn't implement [[Call]])	"object"
Object (native or host and implements [[Call]])	"function"
Object (host)	Implementation-dependent

See also: http://dhtmlkitchen.com/how-property-access-works/