Javascript
Gotchas
Function Constructor
Function constructor can be invoked without using ()
<script> var Const = function(){ this.name = 'Joe'; }; var a = new Const; console.log(a.name); </script>
instanceof Does not Work On Primitive Values
<script> //instanceof does not work on primtive values console.log("" instanceof String); console.log(3 instanceof Number); console.log(false instanceof Boolean); //instanceof works on complex objects console.log(new String() instanceof String); console.log(new Number() instanceof Number); console.log(new Boolean() instanceof Boolean); console.log([] instanceof Array); console.log({} instanceof Object); console.log(/foo/ instanceof RegExp); //note console.log([] instanceof Object); </script>
The typeof Operator Is Limited and Buggy
<script> console.log(typeof ""); console.log(typeof 4); console.log(typeof true); console.log(typeof undefined); //But buggy when it comes to null console.log(typeof null); </script>
Boolean Object Wrappers Are Always true
<script> var b = new Boolean(false); if(b) { console.log("is false"); } else { console.log("is true"); } </script>
Boolean can be called as a function to cast a value to a Boolean
<script> //All below return false console.log(Boolean(undefined)); console.log(Boolean(null)); console.log(Boolean(0)); console.log(Boolean('')); console.log(Boolean(NaN)); //All below return true console.log(Boolean(1)); console.log(Boolean('false')); console.log(Boolean([])); console.log(Boolean({})); console.log(Boolean(function(){})); console.log(Boolean(/foo/)); </script>
Calling Primitive Wrapper Constructors Without new Returns Primitive Values
<script> console.log(String('foo') === 'foo'); console.log(Number(5) === 5); console.log(Boolean('true') === true); // because using new created Object wrapper console.log(new String('foo') === 'foo'); console.log(new Number(5) === 5); console.log(new Boolean('true') === true); </script>
Object() Can Produce Primitive Object Wrappers
<script> console.log(Object(1) instanceof Number); console.log(Object('foo') instanceof String); console.log(Object(true) instanceof Boolean); </script>
Accessing Properties On Primitive Values Fails
<script> //create primtives var N = 5; var S = 'foo'; var B = true; //add properties to primitives, that get destroyed when wrapper is trashed N.test = 'test'; S.test = 'test'; B.test = 'test'; //verify that non of these properties remain. console.log(N.test); //logs undefined console.log(S.test); //logs undefined console.log(B.test); //logs undefined </script>
delete Won't Delete Inherited Properties
<script> var Person = function(){}; //create Person constructor Person.prototype.type = 'person'; //add inherited type property var cody = new Person(); //create instance of Person delete cody.type //delete type console.log(cody.type) //logs person, becuase you can't delete inherited properties </script>
Array.length Can Be Set, And Has Side Effects
<script> //add undefined value to array using length property var myArray1 = []; myArray1.length = 3; console.log(myArray1); //logs [undefined, undefined, undefined] //remove items from array by changing length property myArray2 = [1,2,3,4,5,6,7,8,9] myArray2.length = 1; console.log(myArray2); //log [1] </script>
Credits
Many of the examples taken from:
http://bcreativeweb.blogspot.com
Rouben Meschian
rmeschian@gmail.com