JavaScript Type Stuff

javascript-type-stuff.js

// ============================================================
// Objects vs Primitives
// To make a long story short, use primitives wherever you can.
// ============================================================

// Primitive types: Null, Undefined, Number, Boolean, String.

var num1 = 9000,
    num2 = new Number( 9000 ),
    str1 = "hello world",
    str2 = new String( "hello world" );

num1                // 9000
num2                // 9000

num1 === num2       // false !?

typeof num1         // "number"
typeof num2         // "object" !?

str1                // "hello world"
str2                // "[object Object]" !?

str2 + ""           // "hello world"

str1 === str2       // false !?
str1 === str2 + ""  // true


// ============================================================
// The typeof Operator
// A few things you should probably be aware of.
// ============================================================

typeof 1                    // "number"
typeof "amazing"            // "string"
typeof true                 // "boolean"

typeof {}                   // "object"
typeof []                   // "object" ?!
typeof new Number( 1 )      // "object" ?!
typeof new String( "yo!" )  // "object" ?!
typeof new Boolean( true )  // "object" ?!

typeof function(){}         // "function"
typeof /^foo$/i             // "function" ?!

typeof undefined            // "undefined"
typeof null                 // "object" ?!

typeof NaN                  // "number" ?!


// ============================================================
// The instanceof Operator
// A few more things you should probably be aware of.
// ============================================================

var obj = {};   // sample object
function fn(){} // sample function

obj                         instanceof Object   // true
[]                          instanceof Array    // true
fn                          instanceof Function // true
/^bar$/i                    instanceof RegExp   // true

1                           instanceof Number   // false ?!
"amazing"                   instanceof String   // false ?!
true                        instanceof Boolean  // false ?!

Array( 1, 2, 3 )            instanceof Array    // true
Function( "return 1" )      instanceof Function // true
RegExp( "^bar$", "i" )      instanceof RegExp   // true
Object()                    instanceof Object   // true

Number( 1 )                 instanceof Number   // false ?!
String( "amazing" )         instanceof String   // false ?!
Boolean( true )             instanceof Boolean  // false ?!
Date( "1/1/1999" )          instanceof Date     // false ?!

new Number( 1 )             instanceof Number   // true
new String( "amazing" )     instanceof String   // true
new Boolean( true )         instanceof Boolean  // true
new Date( "1/1/1999" )      instanceof Date     // true
new Array( 1, 2, 3 )        instanceof Array    // true
new Function( "return 1" )  instanceof Function // true
new RegExp( "^bar$", "i" )  instanceof RegExp   // true
new Object()                instanceof Object   // true

// And, for what it's worth...

obj                         instanceof Object   // true
[]                          instanceof Object   // true
fn                          instanceof Object   // true
/^bar$/i                    instanceof Object   // true

1                           instanceof Object   // false
"amazing"                   instanceof Object   // false
true                        instanceof Object   // false

Array( 1, 2, 3 )            instanceof Object   // true
Function( "return 1" )      instanceof Object   // true
RegExp( "^bar$", "i" )      instanceof Object   // true
Object()                    instanceof Object   // true

Number( 1 )                 instanceof Object   // false
String( "amazing" )         instanceof Object   // false
Boolean( true )             instanceof Object   // false
Date( "1/1/1999" )          instanceof Object   // false

new Number( 1 )             instanceof Object   // true
new String( "amazing" )     instanceof Object   // true
new Boolean( true )         instanceof Object   // true
new Date( "1/1/1999" )      instanceof Object   // true
new Array( 1, 2, 3 )        instanceof Object   // true
new Function( "return 1" )  instanceof Object   // true
new RegExp( "^bar$", "i" )  instanceof Object   // true


// ============================================================
// Smarter Type Checking
// Endorsed by the professionals!
// ============================================================

// Since every Object has .toString, and all types inherit from Object...

var obj = {},
    arr = [1,2,3];

obj.toString // "[object Object]"
arr.toString // "1,2,3"


// Use Object .toString method explicitly with call.

var toString = Object.prototype.toString;

toString.call( 1 )                  // "[object Number]"
toString.call( Number( 1 ) )        // "[object Number]"
toString.call( new Number( 1 ) )    // "[object Number]"

toString.call( "a" )                // "[object String]"
toString.call( String( "a" ) )      // "[object String]"
toString.call( new String( "a" ) )  // "[object String]"


// A basic type-checking function.

function type( obj ) {
  var types = {
    "[object Number]":    "number",
    "[object String]":    "string",
    "[object Boolean]":   "boolean",
    "[object Function]":  "function",
    "[object Array]":     "array",
    "[object Date]":      "date",
    "[object RegExp]":    "regexp",
    "[object Object]":    "object"
  };
  return types[ Object.prototype.toString.call( obj ) ];
}

type( 1 )                 // "number"
type( Number( 1 ) )       // "number"
type( new Number( 1 ) )   // "number"

type( "a" )               // "string"
type( String( "a" ) )     // "string"
type( new String( "a" ) ) // "string"

previously -
http://forum.jquery.com/topic/what-about-a-typeof-maybe-instead-of-isarray-isfunction-etc

https://gist.github.com/305222

Yep. This actually comes from a slide deck I have on "JavaScript Gotchas." I just wanted to share the contents with people who might want it.

Thanks for this. Now I don't have to re-open the manual when I forgot this kind of things.

Here is some spec to shed some light on the instanceof stuff.