This is the 9th day of my participation in the August Wen Challenge.More challenges in August

Js writing position

  • inline
<input type="button" value="Button" onclik="alert("hello js"/ >Copy the code
  • Written in the script
<head>
    <script>
        alert("hello js")
    </script>
</head>
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  • Write it in an external JS file and introduce it in the page
<script src="main.js"></script>
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  • Pay attention to
    • Script cannot be written as a single label
    • The loading order is top-down

annotation

  • A single 
    / / comment
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  • Multiple lines 
    / * * /
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Variables (master)

  • The statement
var age
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  • The assignment
var age
age = 18
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  • Declare multiple at the same time
var age, name, sex
age = 10
name = 'you'
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  • Define variables
var age = 10
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  • Define and assign multiple variables simultaneously
var age = 10, name = 'you'
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The data type

  • Five basic data types
    • Number: indicates the value type
    • String: indicates the character type
    • Bollean: Boolean type
    • Undefined: Undefined type
    • Null: empty type
  • Get variable type (master) 
    typeof
    1. typeofThe variable name2. typeof(Variable name)Copy the code
    // typeof variable name/value
console.log(typeof 123);//number
console.log(typeof '123');//string
console.log(typeof true);//boolean
console.log(typeof undefined);//undefined
console.log(typeof null);//object, the object Number type is also called a numeric type.
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The operator

  • Arithmetic operator 
    //+ - * / % 3%2=1
var x = 10;
var y = 5;
console.log(x + y);/ / 15
console.log(x - y);/ / 5
console.log(x * y);/ / 50
console.log(x / y);/ / 2
console.log(10 % 3);/ / 1
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  • Unary operator 
    / front + +var num1 = 8;
++num1;// add 1 to itself
console.log(num1);

/ / rear + +
var num1 = 5;
num1++;// add 1 to itself
console.log(num1);
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  • conclusion
    • Pre ++ : add 1 first, then participate in the operation
    • After ++ : participate in the operation first, then add 1
    • Once the top two are understood, the bottom two make sense
    • Pre – : first minus 1, then participate in the operation
    • Postposition — : participates in the operation first and then subtracts 1

Data type conversion

  • To a numeric type 
    // use parseInt()
// If the first character is a number or an operation symbol, parsing starts until it encounters a non-number character, stops parsing and gets the result
console.log(parseInt("10"));/ / 10
console.log(parseInt("10abcdefg"));/ / 10
console.log(parseInt("g10"));//NaN
console.log(parseInt("1fds0"));/ / 1
console.log(parseInt("10.98"));/ / 10
console.log(parseInt("10.98 abcdefg"));/ / 10
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    // parseFloat() function
// Convert other types to floating point numbers
console.log(parseFloat("10"));/ / 10
console.log(parseFloat("10abcdefg"));/ / 10
console.log(parseFloat("g10"));//NaN
console.log(parseFloat("1fds0"));/ / 1
console.log(parseFloat("10.98.98"));/ / 10.98
console.log(parseFloat("10.98 abcdefg"));/ / 10.98
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    // the Number() function
// You can convert any value to a numeric type, and if there is a non-numeric character in the string to be converted, the result is NaN
console.log(Number("10"));/ / 10
console.log(Number("10abcdefg"));//NaN
console.log(Number("g10"));//NaN
console.log(Number("1fds0"));//NaN
console.log(Number("10.98"));/ / 10.98
console.log(Number("10.98 abcdefg"));//NaN
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    //+, -0
var strNum = '18';
console.log(+strNum);		/ / take is 18
console.log(-strNum);		/ / to 18 take negative
console.log(strNum - 0);     // 18 strNum is converted to a numeric type before subtraction
console.log(strNum + 0);     // 180 is converted to a string type before concatenation
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Note that the plus sign and the minus sign are placed before the variable, respectively, to take the positive and negative effect; Subtract 0 from a variable. By default, convert the variable to a numeric type and subtract 0 from the variable.Copy the code
  • To a string type
    • toString()(master)
    • Stirng()(understand)
Note that the toString() method can only convert numeric and Boolean types to strings, not Undefined and Null. The String() function converts all primitive types to stringsCopy the code

  • To a Boolean type

    • Take the five special values of false 
      ""(Blank string)0(save0  -0)undefined
null
NaN
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    • The others are true

  • The operator

    • The priority of the operator
    Priority from high to bottom1.() has the highest priority2.The unary operator ++ --!3.The arithmetic operators are preceded by * / % and followed by + -4.The relational operator (compare sizes) > >= < <=5.Equality operator (compare equality) ==! = = = =! = =6.Logical operators && before | |7.Assignment operator = += -= *= /= %=Copy the code
    • Ternary operator
    Ex. :var age = 18
var result = age >= 18 ? "Come of age" : "Minor"
console.log(result) Example: Find the maximum value of two valuesvar num1 = 8
var num2 = 2
var max = num1 > num2 ? num1 : num2
console.log(max)
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  • An array of

    • Through the array
    for(var i = 0; i < arr.length; i++){ }Copy the code
    • An array of practice
    Example 1: Find the sum of all elements in an array
  var arr1 = [10.20.30.40.50];
  var sum = 0;
  for(var i = 0; i < arr1.length; i++){
       sum += arr1[i];
  }
  console.log(sum);/ / 150

// Example 2: Find the average of all elements in the array
  var arr2 = [1.2.3.4.5];
  var sum2 = 0;
  for(var i = 0; i < arr2.length; i++){
       sum2 += arr2[i];
  }
  console.log(sum2 / arr2.length);/ / 3
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