Hello, I’m Ken 🦁
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When asked how to learn JavaScript, aken 🦁 decided to contribute his notes from learning JavaScript before.
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2.1_ Data type
2.1.1_ Data types of variables
2.1.2_ Data type classification
2.1.3 _ digital type
The value is a string
2.1.5 _ the Boolean
2.1.6 _undefined and null
2.1.7_ Data type detection
2.1.8 _ literals
2.2_ Data type conversion
2.3 _ operator
2.4_ Branching structure
📚2.1_ Data type
📓2.1.1_ Data type of the variable
JavaScript is a weakly typed language and does not declare the data types of variables in advance. Compared to C and JAVA, these two are strongly typed languages.
// Strong typed language (Java)
int num = 10; // The variable num is an int
Copy the code// Weakly typed languages (JavaScript)
var num = 10; // The variable num is originally a number
num = 'abc'; // Assign a string. Num is now a string
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📒2.1.2_ Data type classification
(1) Number, containing integer values and floating point values:
var num1 = 21; / / integer value
var num2 = 0.21; // Floating-point values
Copy the code(2) Boolean (Boolean), containing the Boolean values true and false:
var bool1 = true; // indicates true, 1, true
var bool2 = false; // Indicates false, 0, and false
Copy the code(3) String, enclosed in single or double quotation marks:
var str1 = ' '; // Empty string
var str2 = 'abc'; // String wrapped in single quotes
var str3 = "abc"; // String wrapped in double quotes
Copy the code(4) Undefined type (Undefined), only one value (Undefined) :
var a; // Declare the variable a with no value assigned, where a is undefined
var b = undefined; // The value of variable b is undefined
Copy the code(5) Stereotypes (Null), only one value is Null:
var a = null; // The value of the variable a is null
Copy the codeNote that the values true, false, undefined, and null in the code are all written in lowercase.
📕 2.1.3 _ digital type
1. Octal numbers are denoted by adding a 0 to the beginning of digits in the base (1)
var num1 = 07;
console.log (num1); // Output: 7
var num2 = 010;
console.log (num2); // Output: 8
Copy the code(2) Add 0x to the beginning of a number to indicate a hexadecimal number
var num1 = 0X9;
console.log (num1); // Output: 9
var num2 = 0Xa;
console.log (num2); // Output: 10
// The hexadecimal values "X" and "a to f" are case insensitive
Copy the code2. Maximum Value The maximum and minimum numeric values are obtained using the following code.
console.log (Number.MAX_VALUE); // output: 1.7976931348623157e+308
console.log (Number.MIN_VALUE) ; // Output: 5e-324
Copy the codeIn the output, you use scientific notation, which you can use in JavaScript to represent numbers.
3. Special values
The numeric type has three special values, Infinity, -infinity, and NaN(Not a Number)
console.log (Number.MAX VALUE * 2); // Output result: Infinity
console.log (-Number.MAX VALUE * 2);// Output: -infinity
console.log('abe' - 100); // Output result: NaN
Copy the codeThe NaN attribute is a special value that represents a non-numeric value. This property is used to indicate that a value is not a number. You can set the Number object to this value to indicate that it is not a numeric value.
To determine if a variable is of a non-numeric type, use isNaN(), which returns a Boolean value with true for non-numeric and false for numeric. The example code looks like this.
console.log (isNaN(12)); // Output result: false
console.log (isNaN('abc')); // Output: true
Copy the code📘2.1.4_ The value is a string
A string is a series of characters used to represent text in a computer. In JavaScript, a string is wrapped in single or double quotes.
var str1 = 'Single quoted string';
var str2 = "Double quoted string";
Copy the code1. Nested single and double quotes
You can write double quotes in a single quoted string or single quotes in a double quoted string. The example code is as follows:
// Correct grammar
var strl = 'I am a "progranmer"'; // I am a"prograrmer";
var str2 = "I'm a 'prograrmer'"; // I'm a 'programmer';
// Common error syntax
var str1 = 'I'm a programmer'; Var str2 = "I ";m a "prograrmer""; Var str3 = 'I am a programmer ';; // Mix single and double quotation marks
Copy the code2. Escape characters
When special characters such as newline and Tab are used in a string, you can use escape characters to escape them. Escape characters all start with “\”.
| Escape character | explain |
|---|---|
| \ ‘ | Single quotes |
| \n | A newline |
| \ ” | Double quotation marks |
3. The value is a string length
var str1 = 'I\'m a programmer';
console.log(str1.length); // Output: 16
// I'm a programmer; // I'm a programmer
var str2 = 'I'm a programmer';
console.log(str2.length); // Output: 5
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4. Access the characters in the string
A string can be accessed in terms of index using the “[index]” syntax. Index starts at 0 and is the length of the string minus 1. If the index maximum is exceeded, undefined is returned.
var str = 'I\'m a programmer' ; // I'm a programmer
console.log(str[0]); // Output result :I
console.log(str[1]); // Output: '
console.log(str[15]); // Output result: r
console.log(str[16]); // The output is undefined
Copy the code5. Case: Output age
// A dialog box is displayed asking the user to enter the age
var age = prompt('Please enter your age');
// Combine statements and output
var msg = 'You this year' + age + 'old';
alert(msg);
Copy the code📙 2.1.5 _ the Boolean
Booleans have two values: true and false
When Boolean (true false) is added to numeric (1, 0)
📗 2.1.6 _undefined and null
If a variable is declared and no value is assigned, then the value of the variable is undefined. We can also assign a null value to a variable. Null is usually used to refer to an empty object pointer, as described in a later section.
Example: Demonstrates the use of undefined and null.
var a;
console.log (a); // The output is undefined
console.log (a + '_'); // Output: undefined_(string)
console.log (a + 1); // Output result: NaN
var b = null;
console.log (b + '_'); // Output: null_(string)
console.log(b + 1); // Output :1(b = 0)
console.log(b + true); // Output :1(b = 0, true = 1)
Copy the code📔2.1.7_ Data type detection
In development, when it is uncertain what data type a variable is, the Typeof operator can be used for data type detection.
Case study:
console.log (typeof 12); // Output result: number
console.log (typeof '12'); // Output result: string
console.log (typeof true); // Output result: Boolean
console.log (typeof undefined); // The output is undefined
console.log (typeof null); // Output result: object
Copy the code🗒 2.1.8 _ literals
A literal is a representation of a fixed value in the source code. Simply put, a literal is used to indicate how the value should be expressed in the code.
📚2.2_ Data type conversion
📓2.2.1. Convert to a string
var num = 3.14;
// Method one uses + "" to concatenate strings
var str = num + ' ';
console.log(str, typeof str); / / 3.14 string
// Method 2 uses toString() to convert to a string
var str = num.toString();
console.log(str, typeof str); / / 3.14 string
// Mode 3 uses String() to convert to a String
var str = String(num);
console.log(str, typeof str); / / 3.14 string
Copy the code> Note: null and undefined cannot be converted using toString() > num = 5; Num.tostring (2) converts 5 to binary, resulting in 101
var num = 5;
console.log ( num.toString(2));/ / 101
Copy the code📒2.2.2. Convert to alphanumeric
// The first method uses parseInt() to convert a string to an integer
console.log ( parsenInt('78'));/ / 78
// Method 2 uses parseFloat() to convert a string to a floating point number
console.log ( parseFloat('3.94'));/ / 3.94
// Mode 3 uses Number() to convert the string to a Number
console.log ( Number('3.94'));/ / 3.94
// Mode 4 implicitly converts using the arithmetic operators (-, *, \)
console.log ('12' - 1); / / 11
Copy the code| To transfer data | Number() and implicit conversion | parseInt() | parseFloat() |
|---|---|---|---|
| Numeric string | Convert to the corresponding number | Convert to the corresponding number | Convert to the corresponding number |
| An empty string | 0 | NaN | NaN |
| A string beginning with a number | NaN | Convert to the beginning number | Convert to the beginning number |
| A string that does not start with a number | NaN | NaN | NaN |
| null | 0 | NaN | NaN |
| undefined | 0 | NaN | NaN |
| false | 0 | NaN | NaN |
| true | 1 | NaN | NaN |
console.log ( parseInt('F'.16));// This code converts the character F to a hexadecimal number, resulting in 15
Copy the code📕2.2.3. Convert to a Boolean
Convert to Boolean using Boolean()
console.log ( Boolean(' '));// false
Copy the code| To transfer data | Boolean() |
|---|---|
| 0 | false |
| NaN | false |
| null | false |
| undefined | false |
| “White” | true |
| 12 | true |
2.3 _ 📚 operator
📓2.3.1. The arithmetic operator \
+、 -、 *、 /、 %
📒2.3.2. Increment (++) and decrement (–) operators \
The _ increment and decrement operators can be written either before or after variables (num++, num–). When placed before a variable, it is called pre-increment (decrement) and when placed after a variable, it is called post-increment (decrement). The difference between pre-increment and post-increment operators is that pre-increment returns the result of a calculation, and post-increment returns the result of a calculation.
var a = 1, b = 1;
console.log (++a); / / 2
console.log (a); / / 2
console.log (b++); / / 1
console.log (b); / / 2
Copy the codevar a = 10;
var b = ++a + 2; // b = 11 + 2, a = 11
var c = b++ + 2; // c = 13 + 2, b = 14
var d = c++ + ++a; // d = 15 + 12, c = 16, a = 12
Copy the codeCase: Demonstration,
// n , n1 = 10
var n = n1++; // n1 = 11 n1++ = 10
console.log ('n = ' + n); // n = 10
console.log ('n1 = ' + n1); // n1 = 11
n = ++n1; // n1 = 12 ++n1 = 12
console.log ('n = ' + n); // n = 12
console.log ('n1 = ' + n1); // n1 = 12
Copy the code📕2.3.3. The comparison operator
| The operator | name |
|---|---|
| < > > = < = | Less than, greater than, greater than or equal to, less than or equal to |
| = = | Is equal to the |
| ! = | Is not equal to |
| = = = | Are congruent |
| ! = = | Not congruent |
console.log (6= =6); // true
console.log (6= ='6'); // true
console.log (6= = =6); // true
console.log (6= = ='6'); // false
console.log (5! = ='5'); // true
Copy the code\
📘2.3.4. Bitwise operators
| The operator | The name of the | The sample | Computing parse |
|---|---|---|---|
| & | with | a&b | If both bits are 1, then 0 is 0 |
| | | or | a|b | If either of the binary bits has a value of 1, the result of the operation is 1 |
| ~ | non | ~ a | 0 is 1,1 is 0 |
| ^ | Exclusive or | a^b | The binary bits are 0 if they are the same, and 1 if they are different |
| << | Shift to the left | a<<b | When we move a to the left by b, the left space will be filled with 0, and the left space will be eliminated |
| >> | Moves to the right | a>>b | When a is shifted to the right by b, the left space is filled with either 0 or 1 according to the sign bit of the original number. The original negative number is filled with 1, and the positive number is filled with 0 |
| >>> | Unsigned right shift | a>>>b | Move a to the right of b, discard the removed bit, and fill the highest bit on the left with 0, regardless of the original number |
// 1. The bit operator ~
var num = 7;varNum2 = ~ num; alert(num2);/ / - 8
var num3 = -1;varNum4 = ~ num3; alert(num4);/ / 0
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
// The underlying principle:
// Start with the basics
// 1. The numbers in the computer are expressed in the form of complement.
// 2. If the value is inverted, 0 becomes 1.
// 3. The first number is a sign bit. 0 is a positive number and 1 is a negative number.
// 4. The original code, the inverse code and the complement of the positive number are the same.
// 5, negative numbers: the sign bit is unchanged, the rest is inverted. Complement: add 1 to the inverse code.
// The inverse complement of 6 and 0 is 0.
// ~ is the inverse of the current number. In the computer, the number is expressed in the form of complement. The number of 7 is the complement of -8, so ~7 == -8;
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/ / use:
// select a random integer~ ~ (Math.random()*7); // Select a random integer from 0 to 6, including 0 and 6~ (Math.random()*7); // Select a random integer from -7 to -1, including -7 and -1~ (Math.random()*7); // Select a random integer from 1 to 7, including 1 and 7
//2), solve abstract leakage
// Abstract leakage: Exposing the details of the underlying implementation in the code.
// For indexOf, check whether indexOf exists, like >-1,! Details such as =-1, >=0, and <0 should be excluded
// It is easy to use ~
var str = 'hello World';if(~str.indexOf('ell'))return true;else
return false;
// 2, by bit and &
var num1 = 25 & 7;var num2 = 25 & -7;var num3 = -25 & 7;var num4 = -25 & -7; alert(num1);/ / 1alert(num2);/ / 25alert(num3);/ / 7alert(num4);/ / - 31Principle:1&1=1,1&0=0,0&1=0,0&0=0. Processing complement according to the rules:25 & 7
/ / 25 & - 7
/ / - 25 & 7
/ / - 25 & - 7
// Red & red == green Green == the complement of the last line
/ / 3, bitwise or |
/ / principle: 1 | 1 | 0 = 1 = 1, 1, 0 | 1 = 1, 0 | 0 = 0.
// Example: refer to the above.
// 4. ^ by bit
// Principle: 1^1=0, 1^0=1, 0^1=1, 0^0=0.
// Example: refer to the above.
Copy the code// 5, left <<, right >>, unsigned right >>
// Simple notation:(a << b) ===Math.ceil(a*(2* * b)); (a >> b) ===Math.floor(a/(2**b));
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📚2.4_ Branch structure
📓 against 2.4.1. If statements
📒 2.4.2. If… Else statements
📕 2.4.3. If… Else if statement
📘 2.4.4. A switch statement
switch(Expression){case 值1Code segment1;
break;
case 值2Code segment2;
break; .default; Code segment n; }Copy the code
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