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.

For example: <source language="javascript"> if(0) { // ToBoolean }

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

Falsy Numbers

-0

+0

NaN

All other numbers boolean convert to true.

Boolean operators type-convert their operands to boolean

&&

||

!

Converting to String

When either operand is a string, the concatenation is performed.

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 primitive, then that value is used. Example:

<source language="javascript"> var o = { valueOf : function() { return 1; } o + 1; // 2. </source> Otherwise, the object's toString is called. <source language="javascript"> var o = { toString : function() { return "1"; } } o + 1; // "11". </source>

Converting to Number

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

Converting to Object

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

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