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Determine and test types using constructor or {}.toString

Package Exports

  • type-of-is

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (type-of-is) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

Description

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

Built in objects / primitives

obj Type.of(obj) Type.is(...) === true
{ x : 2 } Object Type.is(obj, Object)
function () {} Function Type.is(obj, Function)
[1, 2, 3] Array Type.is(obj, Array)
"barf" String Type.is(obj, String)
true Boolean Type.is(obj, Boolean)
10 Number Type.is(obj, Number)
new Date() Date Type.is(obj, Date)
/abc/ RegExp Type.is(obj, RegExp)
new Error("barf!") Error Type.is(obj, Error)

Objects created via new

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((s == t));                    // true
console.log((s === 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));        // [Function: Object]
console.log(Type.of(JSON));        // [Function: Object]


// 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

build status