typeof return value
typeof can determine the basic data types, such as number, string, boolean, undefined, function, object and Symbol 7 types.
The data type of the reference data is imperfect, such as array and null types, and typeof is judged to be object.
Null and Array return object and
use an undefined variable will report an error, but typeof detects a variable data type will not report an error, return undefined
Note: typeof can only distinguish value types
The difference between basic data types and reference types
(1) The storage location is different
Variables of value type will be stored in the stack memory. If a variable of value type is declared in a function, the variable will be automatically destroyed when the function is executed
. The variable name of the reference type will be stored in the stack memory, but the value of the variable will be Stored in the heap memory, the variable of the reference type will not be automatically destroyed. When there is no reference variable to refer to it, the system's garbage collection mechanism will reclaim it
(2) The copy method is different
The direct assignment of a value type variable is a deep copy. For example, var a = 10; var b = a; then the value of a is copied to b. Modifying the value of b will not affect
the variable of a reference type. Direct assignment is actually a pass-by reference. Just a shallow copy
String splicing
Use the plus operator
The easiest way to concatenate strings is to use the plus operator.
Use concat() method
Use the string concat() method to add multiple parameters to the end of the specified string. There is no limit to the type and number of parameters of this method. It will convert all the parameters into strings, and then connect them to the end of the current string in order and finally return the connected string.
The concat() method does not modify the value of the original string, and the operation is similar to the concat() method of the array.
Use join() method
In a specific operating environment, you can also use the join() method of an array to connect strings, such as HTML string output.
Logical operators in Js
There are three logical operators in JavaScript, && and, || or,! NOT. Although they are called logical operators, these operators can be applied to any type of value, not just Boolean values, their results It can also be of any type.