Types
Equality and Comparisons
JavaScript has two different ways of comparing the values of objects for equality.
The Equality Operator
The equality operator consists of two equal signs: ==
JavaScript features weak typing. This means that the equality operator coerces types in order to compare them.
"" == "0" // false
0 == "" // true
0 == "0" // true
false == "false" // false
false == "0" // true
false == undefined // false
false == null // false
null == undefined // true
" \t\r\n" == 0 // true
The above table shows the results of the type coercion, and it is the main reason why the use of == is widely regarded as bad practice. It introduces hard-to-track-down bugs due to its complicated conversion rules.
Additionally, there is also a performance impact when type coercion is in play; for example, a string has to be converted to a number before it can be compared to another number.
The Strict Equality Operator
The strict equality operator consists of three equal signs: ===.
It works like the normal equality operator, except that strict equality operator does not perform type coercion between its operands.
"" === "0" // false
0 === "" // false
0 === "0" // false
false === "false" // false
false === "0" // false
false === undefined // false
false === null // false
null === undefined // false
" \t\r\n" === 0 // false
The above results are a lot clearer and allow for early breakage of code. This hardens code to a certain degree and also gives performance improvements in case the operands are of different types.
Comparing Objects
While both == and === are called equality operators, they behave differently when at least one of their operands is an Object.
{} === {}; // false
new String('foo') === 'foo'; // false
new Number(10) === 10; // false
var foo = {};
foo === foo; // true
Here, both operators compare for identity and not equality; that is, they will compare for the same instance of the object, much like is in Python and pointer comparison in C.
In Conclusion
It is highly recommended to only use the strict equality operator. In cases where types need to be coerced, it should be done explicitly and not left to the language's complicated coercion rules.
The typeof Operator
The typeof operator (together with instanceof) is probably the biggest design flaw of JavaScript, as it is almost completely broken.
Although instanceof still has limited uses, typeof really has only one practical use case, which does not happen to be checking the type of an object.
Note: While typeof can also be called with a function like syntax, i.e. typeof(obj), this is not a function call. The parentheses behave as normal and the return value will be used as the operand of the typeof operator. There is no typeof function.
The JavaScript Type Table
Value Class Type
-------------------------------------
"foo" String string
new String("foo") String object
1.2 Number number
new Number(1.2) Number object
true Boolean boolean
new Boolean(true) Boolean object
new Date() Date object
new Error() Error object
[1,2,3] Array object
new Array(1, 2, 3) Array object
new Function("") Function function
/abc/g RegExp object (function in Nitro/V8)
new RegExp("meow") RegExp object (function in Nitro/V8)
{} Object object
new Object() Object object
In the above table, Type refers to the value that the typeof operator returns. As can be clearly seen, this value is anything but consistent.
The Class refers to the value of the internal [[Class]] property of an object.
From the Specification: The value of [[Class]] can be one of the following strings.Arguments, Array, Boolean,Date, Error, Function,JSON, Math, Number,Object, RegExp, String.
In order to retrieve the value of [[Class]], one has to make use of the toString method of Object.prototype.
The Class of an Object
The specification gives exactly one way of accessing the [[Class]] value, with the use of Object.prototype.toString.
function is(type, obj) {
var clas = Object.prototype.toString.call(obj).slice(8, -1);
return obj !== undefined && obj !== null && clas === type;
}
is('String', 'test'); // true
is('String', new String('test')); // true
ES5 Note: For convenience the return value of Object.prototype.toString for both null and undefined was changed from Object to Null and Undefined in ECMAScript 5.
In the above example, Object.prototype.toString gets called with the value of this being set to the object whose [[Class]] value should be retrieved.
Testing for Undefined Variables
typeof foo !== 'undefined'
The above will check whether foo was actually declared or not; just referencing it would result in a ReferenceError. This is the only thingtypeof is actually useful for.
In Conclusion
In order to check the type of an object, it is highly recommended to use Object.prototype.toString because this is the only reliable way of doing so. As shown in the above type table, some return values of typeof are not defined in the specification; thus, they can differ between implementations.
Unless checking whether a variable is defined, typeof should be avoided.
The instanceof Operator
The instanceof operator compares the constructors of its two operands. It is only useful when comparing custom made objects. Used on built-in types, it is nearly as useless as the typeof operator.
Comparing Custom Objects
function Foo() {}
function Bar() {}
Bar.prototype = new Foo();
new Bar() instanceof Bar; // true
new Bar() instanceof Foo; // true
// This just sets Bar.prototype to the function object Foo,
// but not to an actual instance of Foo
Bar.prototype = Foo;
new Bar() instanceof Foo; // false
Using instanceof with Native Types
new String('foo') instanceof String; // true
new String('foo') instanceof Object; // true
'foo' instanceof String; // false
'foo' instanceof Object; // false
One important thing to note here is that instanceof does not work on objects that originate from different JavaScript contexts (e.g. different documents in a web browser), since their constructors will not be the exact same object.
In Conclusion
The instanceof operator should only be used when dealing with custom made objects that originate from the same JavaScript context. Just like the typeof operator, every other use of it should be avoided.
Type Casting
JavaScript is a weakly typed language, so it will apply type coercion wherever possible.
// These are true
new Number(10) == 10; // Number.toString() is converted
// back to a number 10 == '10'; // Strings gets converted to Number
10 == '+10 '; // More string madness
10 == '010'; // And more
isNaN(null) == false; // null converts to 0
// which of course is not NaN // These are false
10 == 010;
10 == '-10';
ES5 Note: Number literals that start with a 0 are interpreted as octal (Base 8). Octal support for these has been removed in ECMAScript 5 strict mode.
To avoid the issues above, use of the strict equal operator is highly recommended. Although this avoids a lot of common pitfalls, there are still many further issues that arise from JavaScript's weak typing system.
Constructors of Built-In Types
The constructors of the built in types like Number and String behave differently when being used with the new keyword and without it.
new Number(10) === 10; // False, Object and Number
Number(10) === 10; // True, Number and Number
new Number(10) + 0 === 10; // True, due to implicit conversion
Using a built-in type like Number as a constructor will create a new Number object, but leaving out the new keyword will make the Number function behave like a converter.
In addition, passing literals or non-object values will result in even more type coercion.
The best option is to cast to one of the three possible types explicitly.
Casting to a String
'' + 10 === '10'; // true
By prepending an empty string, a value can easily be cast to a string.
Casting to a Number
+'10' === 10; // true
Using the unary plus operator, it is possible to cast to a number.
Casting to a Boolean
By using the not operator twice, a value can be converted a boolean.
!!'foo'; // true
!!''; // false
!!'0'; // true
!!'1'; // true
!!'-1' // true
!!{}; // true
!!true; // true