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(Converting to String)
(Converting to Object)
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// undefined.toString(); // TypeError
// undefined.toString(); // TypeError
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Revision as of 21:40, 5 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.


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:

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

Falsy Numbers: +/-0 and 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 a primitive value, then that value is used. Example:

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

Otherwise, 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.

Converting 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:

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