What you don't know about JavaScript (1)

Confusions 1:

Let's take a look at an example:

function test(){message = "hi";}test();alert(message);

The output string "hi"

In a function, the variable defined by var is a local variable, and the variable omitted by the var operator is a global variable.

Confusions 2:

alert(undefined == null)

The result is "true"

We know that js contains basic types and reference types, including number, string, boolean, undefined, and null. if a basic type is not initialized, it will be of the undefined type, and null indicates a null pointer. In fact, the undefined value is derived from null, so true is returned.

Confused 3:

Alert (isNaN (NaN); // truealert (isNaN (10); // falsealert (isNaN ("10 ")); // false automatic type conversion alert (isNaN ("blue"); // truealert (isNaN (true); // false automatic type conversion

NaN indicates that a non-a Number is a special value. In ECMAScript, if any value is divided by 0, NaN is returned, and any operation involving NaN will return NaN. naN is different from any value, including itself.

alert(NaN == NaN);  // false 

Confused 4:

for(var propName in window){document.write(propName);}

The For-in statement is a precise iterative statement that can be used to enumerate the attributes of an object, similar to for (String s: String []) in java.

Confusions 5:

Var qs = location. search. substring (1); var hostName = location. hostname; var url = location. href; // equivalent to the following with (location) {var qs = search. substring (1); var hostName = hostname; var url = href ;}

The with statement is used to set the code scope to a specific object to simplify the work of writing the same object multiple times.

Puzzles 6:

function howManyArgs(){alert(arguments.length);}howManyArgs("String", 45);  //2howManyArgs();  //0howManyArgs(12);   //1

The parameters in ECMAScript are represented by an array internally. The function always receives this array, regardless of the parameters contained in the array (if no parameter exists ).

Naming parameters are only convenient, but not necessary. Let's look at the example below.

function doAdd(num1, num2){arguments[1] = 10;alert(arguments[0] + num2);}

Because the values in the arguments object are automatically reflected in the corresponding named parameters, num2 is changed to 10, but their memory space is independent (not referenced ), in addition, the length of the arguments object is determined by the number of input parameters, not by the number of named parameters when defining the function.

Puzzles 7:

For (var I = 0; I <10; I ++) {} alert (I); // The output result is 10.

JavaScript has no block-level scope.