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• Number • String • Boolean • Null • Undefined • Object There are five primitive types in JavaScript. Everything else is an object.


Various operations in JavaScript require conversion between primitive and object values.

Converting to Boolean

When evaluating any expression that requires a boolean value, a boolean value must be produced. This occurs with the internal [[ToBoolean]] § 9.2).

For example:

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

All numbers boolean-convert to true except for +/-0 and NaN.

Boolean operators use type-conversion for the evaluation of their operand or operands.

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

All falsy values:


All other primitive values and all objects are truthy.

Converting to String

With the + operator, when either operand is a string, concatenation is performed.

All native objects have a toString method. Number.prototype.toString(base) is special in that it takes a base parameter.

String(15); // Calls ToPrimitive(input argument, hint String).

Conversion to Primitive

Mathematical unary and binary operators, as well as boolean operators require the operands to be converted to primitives.

The Addition operator is used, the operands must be converted into primitive values. First, the interpreter calls the object's valueOf. If the result is a primitive value, then that value is used. Example:

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

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

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

Example: toString, valueOf, and concatenation

Converting to Number

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

var d1 = new Date(NaN);
var d2 = new Date(NaN);
d1 >= d2; // true, conversion to number.
d1 <= d2; // true, conversion to number.
d1 == d2 // false, different objects.

parseInt(s, radix)

To force use of a particular base, use the radix parameter:

parseInt("09", base) // base from 2 to 36.

If radix is omitted, the base is determined by the contents of the string. Any string beginning with 0x or 0X represents a hexadecimal number. A string beginning with a leading 0 may, in older implementations, be parsed as octal (as if raxix were 8), in ECMA-262 Ed 3 (octal digits are 0-7). If string 09 is converted to 0.

var t = "0xf";
Number(t); // 15
+t;        // 15

Primitive to Object with Property Accessors . and []

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

// Primitive to Object conversion.
true.toString(); // creates a temporary Boolean Object.
1.2.valueOf();   // creates a temporary Number object.
" foo ".trim();  // creates a temporary String Object.
// null.toString(); // TypeError
// undefined.toString(); // TypeError

See also:

Object Creation

// You'll never need these.
new Object(1);    // Results a Number object.
new Object(true); // Results a Boolean object.
new Object("");   // Results a String object.
JSON.parse("{}"); // Results an Object object.

Type Checking

Use sparingly. Avoid overloading with typechecking.

The typeof Operator

typeof someval;
Type Result
Undefined "undefined"
Null "object"
Boolean "boolean"
Number "number"
String "string"
Object (native or host and doesn't implement [[Call]]) "object"
Object (native or host and implements [[Call]]) "function"
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