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.

In order to avoid the 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, having literals or non-object values in there 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 a empty string a value can easily be casted 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'; // false
!!'1'; // true
!!'-1' // true
!!{}; // true
!!true; // true

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Intro	Objects	The prototype
hasOwnProperty	Functions	How this works
Closures and references	The arguments object	Scopes and namespaces
Constructors	Equality and comparisons	Arrays
The Array constructor	The for in loop	The typeof operator
The instanceof operator	Type casting	undefined and null
Reasons against eval	setTimeout and setInterval	Automatic semicolon insertion