type-of-is

Determine and test types using constructor or {}.toString

Description

Sensible / unsurprising JavaScript type detection and comparison using a combination of ({}).toString and constructors.

objType.of(obj)Type.is(...) === true
{ x : 2 }ObjectType.is(obj, Object)
function () {}FunctionType.is(obj, Function)
[1, 2, 3]ArrayType.is(obj, Array)
"barf"StringType.is(obj, String)
trueBooleanType.is(obj, Boolean)
10NumberType.is(obj, Number)
new Date()DateType.is(obj, Date)
/abc/RegExpType.is(obj, RegExp)
new Error("barf!")ErrorType.is(obj, Error)
function Person (name) {
  this.name = name;
}
Person.prototype.barf = function () {
  return this.name + " just barfed!";
};
 
var ralph = new Person('Ralph');
 
Type.of(ralph);                  // [Function: Person] 
Type.is(ralph, Person);          // true 
Type.is(ralph, Object);          // false 
Type.instance(ralph, Person);    // true 
Type.instance(ralph, Object);    // true 

Latest Version

3.4.0

Installation

npm install type-of-is

or in package.json

{
  ...
  "dependencies": {
    "type-of-is": "~3.4.0"
  }
}

Usage

var Type = require('type-of-is');
 
Type.of(obj);              // returns constructor type of an object 
Type.string(obj);          // provides type as String 
Type.is(obj, type);        // tests whether obj is of type (constructor or String) 
Type.instance(obj, type);  // wrapper of "obj instanceof type" 
Type.extension(SomeCoffeeScriptClassThatExtendsBarf, SomeCoffeeScriptClassNamedBarf)
 
// The top level Type export delegates to Type.of or Type.is based on argument count 
 
Type(obj) === Type.of(obj);
Type(obj, type) === Type.is(obj, type);

More examples

var Type = require('type-of-is');
 
// Type.of(arg) and Type(one_argument) return constructor of type 
console.log(Type.of('hi there ok'));  // [Function: String] 
console.log(Type.of(342));            // [Function: Number] 
console.log(Type.of({}));             // [Function: Object] 
console.log(Type.of([1, 2, 3]));      // [Function: Array] 
console.log(Type.of(null));           // null 
console.log(Type.of(undefined));      // undefined 
console.log(Type(true));              // [Function: Boolean] 
console.log(Type(function () {}));    // [Function: Function] 
console.log(Type(/abcd/));            // [Function: RegExp] 
console.log(Type(new Date()));        // [Function: Date] 
console.log(Type(new Error()));       // [Function: Error] 
 
// Type.string(arg) returns the string name of constructor 
console.log(Type.string('hi there ok'));  // "String" 
console.log(Type.string(342));            // "Number" 
console.log(Type.string({}));             // "Object" 
console.log(Type.string([1, 2, 3]));      // "Array" 
console.log(Type.string(null));           // "Null" 
console.log(Type.string(undefined));      // "Undefined" 
console.log(Type.string(true));           // "Boolean" 
console.log(Type.string(function () {})); // "Function" 
console.log(Type.string(/abcd/));         // "RegExp" 
console.log(Type.string(new Date()));     // "Date" 
console.log(Type.string(new Error()));    // "Error" 
 
// Type.is(object, type) and Type(object, type) tests object type 
console.log(Type.is(true, Boolean));      // true 
console.log(Type.is("1231", Number));     // false 
console.log(Type.is("1231", String));     // true 
console.log(Type.is("1231", "String"));   // true 
console.log(Type.is("1231", Object));     // false 
console.log(Type([], Object));            // false 
console.log(Type({}, Object));            // true 
console.log(Type([], Array));             // true 
console.log(Type(new Date(), Date));      // true 
console.log(Type(new Date(), Object));    // false 
 
var s = "hihihi";
var Stringy = Type.of(s);
var t = new Stringy("hihihi");
console.log((== t));                    // true 
console.log((=== t));                   // false 
 
 
// User defined objects should be instances of Objects but also can get actual constructor type 
function Person (name) {
  this.name = name;
}
Person.prototype.barf = function () {
  return this.name + " just barfed!";
};
 
var ralph = new Person('Ralph');
 
console.log(Type.of(ralph));                 // [Function: Person] 
console.log(Type.is(ralph, Person));         // true 
console.log(Type.is(ralph, Object));         // false 
console.log(Type.instance(ralph, Person));   // true 
console.log(Type.instance(ralph, Object));   // true 
 
 
// arguments is weird edge case, there's no Arguments global but typeof arguments is "arguments" 
// type returned is Object, but not sure what would be preferable 
(function () {
  console.log(Type.of(arguments));          // [Function: Object] 
})();
 
 
// other built-ins 
console.log(Type.of(Infinity));    // [Function: Number] 
console.log(Type.of(-Infinity));   // [Function: Number] 
console.log(Type.of(NaN));         // [Function: Number] 
console.log(Type.of(Math));        // {} 
console.log(Type.of(JSON));        // {} 
 
 
// Returning constructor as type allows it to be used to create new objects i.e. 
var s = "s";
var t = new Type.of(s)("t");
console.log(t.toUpperCase());   // "T" 
 
// Type.any(obj, [Array, Of, Types]) and Type(obj, [Array, Of, Types]) should test whether 
// the object is any of the passed in types 
var str = 'hihihi';
console.log(Type.any(str, [String, Number, Array])); // true 
console.log(Type(str, [Array, RegExp]));             // false 
 
// multi-frame dom 
var iFrame = document.createElement('IFRAME');
document.body.appendChild(iFrame);
var IFrameArray = window.frames[0].Array;
var array = new IFrameArray();
 
console.log(array instanceof Array);        //false 
console.log(array instanceof IFrameArray);  //true; 
console.log(Type.of(array));                // Array 
console.log(Type.is(array, Array));         // false 
console.log(Type.is(array, "Array"));       // true 
 

Rationale

Try to iron over some of the surprises in JavaScript type detection

  1. typeof is unreliable / surprising in multiple cases (Array -> object, null -> object, etc.)

  2. constructor checking is unreliable in multi-frame dom environments

  3. type comparison using strings whose string case / formatting differs from constructor names introduces unnecessary complexity

  4. ({}).toString returns "[object Object]" for objects created via new rather than constructor name called with new

Links

http://ecma262-5.com/ELS5_HTML.htm

http://javascriptweblog.wordpress.com/2011/08/08/fixing-the-javascript-typeof-operator/

http://skilldrick.co.uk/2011/09/understanding-typeof-instanceof-and-constructor-in-javascript/

http://javascriptweblog.wordpress.com/2010/09/27/the-secret-life-of-javascript-primitives/

http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/

http://www.2ality.com/2011/11/improving-typeof.html

TODO

check back on https://github.com/ariya/phantomjs/issues/11722

#Build status