How is almost everything in Javascript an object?

by

No, not everything is an object in JavaScript. Many things that you interact with regularly (strings, numbers, booleans) are primitives, not objects. Unlike objects, primitive values are immutable. The situation is complicated by the fact that these primitives do have object wrappers (String, Number and Boolean); these objects have methods and properties while the primitives do not, but the primitives appear to have methods because JavaScript silently creates a wrapper object when code attempts to access any property of a primitive.

For example, consider the following code:

var s = "foo";
var sub = s.substring(1, 2); // sub is now the string "o"

Behind the scenes, s.substring(1, 2) behaves as if it is performing the following (approximate) steps:

  1. Create a wrapper String object from s, equivalent to using new String(s)
  2. Call the substring() method with the appropriate parameters on the String object returned by step 1
  3. Dispose of the String object
  4. Return the string (primitive) from step 2.

A consequence of this is that while it looks as though you can assign properties to primitives, it is pointless because you cannot retrieve them:

var s = "foo";
s.bar = "cheese";
alert(s.bar); // undefined

This happens because the property is effectively defined on a String object that is immediately discarded.

Numbers and Booleans also behave this way. Functions, however, are fully-fledged objects, and inherit from Object (actually Object.prototype, but that’s another topic). Functions therefore can do anything objects can, including having properties:

function foo() {}
foo.bar = "tea";
alert(foo.bar); // tea

More Related Contents:

  1. Why is it necessary to set the prototype constructor?
  2. Advantages of using prototype, vs defining methods straight in the constructor? [duplicate]
  3. Javascript object members that are prototyped as arrays become shared by all class instances
  4. The value of “this” within the handler using addEventListener
  5. Anonymous class instance —- is it a bad idea?
  6. Class vs. static method in JavaScript
  7. Benefits of prototypal inheritance over classical?
  8. JavaScript: Class.method vs. Class.prototype.method
  9. prototype based vs. class based inheritance
  10. Understanding prototypal inheritance in JavaScript
  11. Adding custom properties to a function
  12. Get functions (methods) of a class [duplicate]
  13. Difference of the value, prototype and property
  14. JavaScript private methods
  15. Problems with circular dependency and OOP in AngularJS
  16. setTimeout() inside JavaScript Class using “this”
  17. How to call a parent method from child class in javascript?
  18. Javascript dynamically invoke object method from string
  19. How to dynamically set a function/object name in Javascript as it is displayed in Chrome
  20. Object vs Class vs Function
  21. Define Private field Members and Inheritance in JAVASCRIPT module pattern
  22. ES6 – Call static method within a class
  23. Preserve ‘this’ reference in javascript prototype event handler [duplicate]
  24. Typescript: can not access member value in inherited class constructor
  25. Can you alter a Javascript function after declaring it?
  26. Duck Typing in Javascript
  27. OO Javascript constructor pattern: neo-classical vs prototypal
  28. Split a Javascript class (ES6) over multiple files?
  29. How to access a method from a class from another class?
  30. How to set javascript private variables in constructor?

Leave a Comment