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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:

var n = 0;
if(n) { // false
var t = !""; // true. Empty string is falsy.
var f = !"f"; // false. 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;            // 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

When either operand is a string, concatenation is performed.

Object to Primitive

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.

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

Type Checking

The typeof Operator

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

See also:

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