JavaScript/Notes/TypeConversion

From Noisebridge
< JavaScript(Difference between revisions)
Jump to: navigation, search
(Converting to String)
(Converting to String)
Line 51: Line 51:
 
</source>
 
</source>
  
Otherwise, if valueOf results in an object &mdash;and Object.prototype.valueOf does &mdash; the object's toString is called.  
+
Otherwise, if <code>''o''.valueOf</code> results in an object &mdash;and <code>Object.prototype.valueOf</code> does &mdash; the object's <code>toString</code> is called.  
 
<source lang="javascript">
 
<source lang="javascript">
 
var o = { toString : function() { return "1"; } };
 
var o = { toString : function() { return "1"; } };

Revision as of 01:03, 6 January 2014

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

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

Contents

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:

var n = 0;
if(n) { // false
}
 
var t = !""; // Empty string is falsy.
var f = !"f"; // Non-empty strings are not falsy.

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.

1 && 0;            // 1.
"" || 0;           // 0.
null || undefined; // undefined.
undefined || 1;    // 1.
NaN || 0;          // 0;

All falsy values:

false
""
null
undefined
0
NaN

All other primitive values and all objects are truthy.

Converting to String

When either operand is a string, 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 a primitive value, then that value is used. Example:

var o = { 
  valueOf : function() { return 1; } 
};
o + 1; // 2.

Otherwise, if o.valueOf results in an object —and Object.prototype.valueOf does — the object's toString is called.

var o = { toString : function() { return "1"; } };
o + 1; // "11".

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.

Primitive to Object

Property access operation on string, number, and boolean primitives results in the creation of a temporary object.

true.toString(); // Boolean Object.
1.2.valueOf(); // Number object.
" foo ".trim(); // String Object.
 
// null.toString(); // TypeError
// undefined.toString(); // TypeError

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

Personal tools