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"> if(0) { // false }
var t = !""; // Empty string is falsy. var f = !"f"; // Non-empty strings are not falsy. </source>
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 lang="javascript"> var o = { valueOf : function() { return 1; } o + 1; // 2. </source>
Otherwise, 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 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 lang="javascript"> true.toString(); // Boolean Object. 1.2.valueOf(); // Number object. " foo ".trim(); // String Object.
// null.toString(); // TypeError // undefined.toString(); // TypeError </source>