Javascript Base Point

1. Basic types

There are several basic data types for Javascript:

1. Number
2. String:
3. Boolean:
4. Undefined: When you access a non-existent variable, you get it undefined . It is also available when you define a variable but do not assign it a value undefined . JavaScript automatically initializes the variable to that value, and the undefined type has only one value undefined .
5. Null: means that the variable has no value or is empty.

Except for the above type, the other types are object

typeofreturn value

• "Number"
• "String"
• "Boolean"
• "Undefined"
• "Object"
• "Function"

Numeric prefixes

// Octal, hexadecimal numbersvar0377;  // octalvar0x9F;  // hex// Exponent literalsvar2e3;   // 2000var2e-3;  // 0.002

Infinity

Infinity is of type number, and JavaScript can handle a maximum value of 1.7976931348623157e+308 and a minimum of 5e-324.

varA =6/0;//Infinityvari =-Infinity;//-infinityvarm =Infinity–Infinity;//NaNvarn =-Infinity+Infinity;//NaNvarN1 =Infinity– -;//InfinityvarN2 =-Infinity*3;//-infinityvarN3 =Infinity/2;//InfinityvarN4 =Infinity–99999999999999999;//Infinity

NaN

NaNis also a special number value:

vartypeofNaN;  // "number"varNaN;         // NaNvar10"f";    // NaNvar12NaN// NaN

2. String type Conversion

var‘1‘3 * s;typeof s;// "number"s;// 3var‘1‘;s++;typeof s;// "number"s;// 2

3. Logical Judgment

Any value can be given a Boolean value with two backslashes. In addition to the following several return false, most values return true:

• Empty string ""
• Null
• undefined
• Number 0
• DigitalNaN
• Boolean false

Note the string "0", "", and "false" to true when converting to a Boolean value

4. Comparison characters

1==1;//True1==2;//False1==' 1 ';//True1===' 1 ';//False1===1;//True1!=1;//False1!=' 1 ';//False1!=' 2 ';//True1!==1;//False1!==' 1 ';//TrueNaN==NaN;//False//NaN is not equal to any value, including his own

Undefined and Null

Foo//Referenceerror:foo is not definedtypeofFoo//"undefined"varSomevar;somevar;typeofSomevar;//"undefined"//JavaScript automatic initialization variable value is undefinedvarSomevar;somevar = = =undefined;//True//JavaScript does not automatically assign null initial values to variablesvarSomevar =NULL;//nullSomevar;//nulltypeofSomevar;//"Object"//undefined & nullvari =1+undefinedI//NaNvari =1+NULLI//1//Convert to a number:1*undefined;//NaN1*NULL;//0//Convert to a Boolean value:!!undefined;//False!!NULL;//False//Convert to a string:"Value:"+NULL;//"Value:null""Value:"+undefined;//"value:undefined"

5. Array Arrays

varA = [1,2,3];//You can add items to the array in the following waysa[3] =' Four ';//"four"//If you add an item this way, items that were not specified before this index will be initially undefineda[6] =' new ';//"new"A//[1, 2, 3, "Four", Undefined, Undefined, "new"]//Use Delete to remove itemsvarA = [1,2,3];Deletea[1];//TrueA//[1, Undefined, 3]typeofa[1];//"undefined"the//array can contain multiple types of data and can even contain another arrayvarA = [[1,2,3], [4,5,6]];a;//[array[3], array[3]

6. Code blocks check for variable existence

varresult ="";if(typeofSomevar!=="undefined") {result ="Yes";} Result// ""varSomevar;if(typeofSomevar!=="undefined") {result ="Yes";} Result// ""Somevar =undefined;if(typeofSomevar!=="undefined") {result ="Yes";} Result// ""Somevar =123;if(typeofSomevar!=="undefined") {result =' yes ';} Result//"Yes"

Javascript Base Point