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 the expression of an if statement the Javascript interpreter will type-convert the result of that expression to boolean in order to make its decision. Also various operators internally type-convert their operands to boolean in order to determine what action to take. These include the logical operators like AND (&&), OR (||) and NOT (!). The NOT operator type-converts its operand to boolean and if that value is boolean true it returns false and if false it returns true. As the result of a NOT operation is a boolean value that is the inverse of the type-converted true-ness of its operand, two NOT operations together will return a boolean value that is equivalent to the result of type-converting the operand 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>