There are two data types in ECMAScript: basic data type and reference data type.

Basic data type

Basic data types are also called simple data types, which are Undefined, Null, Boolean, Number, String and Symbol.

The basic data type is characterized by a fixed size of space in memory and stored on the stack.

1. Undefined type

The undefined type has only one value, undefined. If a variable is declared without initialization, the value of the variable is undefined.

var message;
console.log(message); // undefined

var example; 
console.log(example); // undefined

 

2. Null type

The null type has only one value, null. Null represents a null object pointer.

As long as the variable that saves the object does not really save the object, you should let the variable save null values, so that you can further distinguish between null and undefined.

var obj = null;
alert(obj); // null

 

3. Boolean type

Boolean has only two values: true and false. Case sensitive, true and false are not Boolean values.

var boo = true;
alert(obj); // true

var lost = false;
alert(lost); // false

 

The conversion rules between Boolean type and data type are as follows:

data type	Value converted to true	Value converted to false
Boolean	true	false
Undefined	Not applicable	undefined
String	Any non empty string	Empty string ('')
Number	Any non-zero numeric value	0 and NaN
Object	Any object	null

 

4. String type

The String type represents a String, which can be expressed in single quotation marks (') or double quotation marks (").

var a = 'a'; // Effective
var b = "a"; // Effective

 

Once strings are created, their values cannot be changed. If you change the value of the string, destroy the original value, and then fill the variable with the string with the new value.

var lang = "Java";
lang = lang + "Script";
console.log(lang); // JavaScript

You can convert to a string using toString().

var age = 11;
age.toString(); // "11"

　

Note: if you don't know whether the converted value is null or undefined, you can use string (). The String() function follows the following conversion rules:

• If the value has a toString() method, call this method to return the corresponding result;
• If the value is null, return "null";
• If the value is undefined, return "undefined";

 

5. Number type

The Number type is divided into integer and floating point numbers. Numerical values can be expressed in decimal, octal and hexadecimal.

The range of values that ECMAScript can save can be number MIN_ Value and number MAX_ Value.

5.1 floating point value

var floatNum = 1.1;

var floatNum1 = 0.1;
var floatNum2 = .1; // Valid, but not recommended 

var floatNum3 = 1.; // Resolve to 1

var floatNum4 = 3.145e7; // Resolved to 3145000, e Means 3.145 * (10 (7th power of)

　　

5.2. Number, parseInt, parseFloat value conversion

parseInt can convert a value to an integer, and parseFloat can convert a value to a floating point number.

var num1 = parseInt("11.5"); // 11
var num2 = parseFloat("2.5"); // 2.5

 

5.3 value range

Value range available number MIN_ Value and number MAX_ Value = indicates;

 

　　5.4,NaN

NaN is a non numeric value and is a special value. Indicates that an operation book that originally wants a value does not return a value. NaN and any value do not want to wait, including NaN itself.

alert(NaN == NaN); // false

isNaN() will judge whether the number is a value. After receiving a parameter, it will convert the value to a value. If it cannot be converted to a numeric value, it returns true.

alert(isNaN(NaN)); // true
alert(isNaN(10)); // false，10 Is a numeric value
alert(isNaN("10")); // false，Can be converted to a value of 10
alert(isNaN("blue")); // true，Cannot be converted to a numeric value
alert(isNaN(true)); // false,Converted to value 1

 

6,Symbol

The object attribute names in ES5 are strings, which can easily lead to attribute name conflicts. If there is a mechanism to ensure that the attribute name is unique, Symbol is introduced in ES6.

Symbol is an original basic data type newly added in ES6, representing unique values. The symbol value is generated by the symbol function and cannot be used as a constructor.

const symbol1 = Symbol();
const symbol2 = Symbol(42);
const symbol3 = Symbol('foo');

console.log(typeof symbol1);// "symbol"
console.log(symbol2 === 42);//  false
console.log(Symbol('foo') === Symbol('foo'));// false

 

Reference data type

In ECMAScript, a reference data type is a data structure that organizes data and functions together.

1. Object type

So far, most reference type values are instances of Object type.

// 1,use new Heel Object Constructor
var person = new Object();
person.name = "Tom";
// 2,Object literal representation
var person = {
    name: "Tom",
    age:12
}
console.log(person.name); // Tom
console.log(person.age); // 12

var person1 = {};
person1.name = "Tom";
console.log(person1.name); // Tom
console.log(person1.age); // undefined

 

2. Array type

Array type is one of the most commonly used reference types.

a) method for detecting numerical elements: forEach ， some ， every ， map ， filter

b) methods of changing array length: pop and push (stack method), unshift and shift (queue method), slice splice concat

var a = [1, 2, 3];
var b = a.pop(); //b=3,a=[1,2],pop()Returns the last item of the array, so b The value of is an array a Last item
var c = a.push(4); //c=3, [1,2,4]，c yes a.length

var d = a.unshift("red"); //d=4, ["red",1,2,4],d by a.length
var e = a.shift(); //e="red",a=[1,2,4],e Is the first item of the removed array and returns that item, the array a The length of the should be reduced by 1
var sliceA = a.slice(1, 2); //sliceA=2,4,  a=[1,2,4]
var spliceA = a.splice(0, 1);//spliceA=1, a=[2,4]

 

c) Element search: findIndex, lastFindIndex, indexOf and lastIndexOf (location method), filter includes

var a = [1, 2, 3, 2];
alert(a.indexOf(2)); // 1
alert(a.lastIndexOf(2)); // 3 

 

d) Sort: sort reverse

var a = [2, 1, 3, 5, 4];
a.sort(); // 1,2,3,4,5
b.reverse(); // 5,4,3,2,1

 

e) Type conversion or detection: join toString isArray

var a = [1, 2, 3, 2];
Array.isArray(a); // true
a instanceof Array //true,instanceof Non array method
a.toString(); //'1,2,3,2'，a=[1, 2, 3, 2]
a.join(); //'1,2,3,2'，a=[1, 2, 3, 2]

 

f) Merge method: reduce} reduceRight

Both methods iterate over all items and return the final value. The functions of the two methods are the same, but the directions are different.

reduce() traverses from the first item to the last, and reducereight() traverses from the last item to the first item.

var a = [1, 2, 3, 2];
var b = a.reduce((x, y) => {
    return x + y
});//b=8; 
var c = a.reduceRight((x, y) => {
    return x + y
});//c=8; 

 

3. Date type

List some common methods to obtain time:

getDate()	Returns the day of the month (from 1 to 31).
getDay()	Returns the day of the week (0-6).
getFullYear()	Returns the year.
getHours()	Return hours (0-23).
getMinutes()	Returns minutes (0-59).
getMonth()	Returns the month (0-11).
getSeconds()	Returns the number of seconds (0-59).
getTime()	Returns the number of milliseconds since midnight on January 1, 1970, and the specified date.
now()	Returns the number of milliseconds of the date and time when this method was called

4. RegExp type

 

exec	A RegExp method that finds a match in a string, which returns an array (null if it does not match).
test	A RegExp method that tests for a match in a string and returns true or false.
match	A String method that finds a match in a String. It returns an array and returns null when it does not match.
matchAll	A String method that finds all matches in a String and returns an iterator.
search	A String method that tests for a match in a String, which returns the location index to match, or - 1 if it fails.
replace	A String method that performs a search for a match in a String and replaces the matched substring with a replacement String.
split	A String method that uses a regular expression or a fixed String to separate a String and stores the separated substrings in the array.

View common regular expression .

5. Function type

Please check This article.

 

6. Map type

 

7. Set type

 

8. Basic package type

Boolean type (not recommended):

The usage method is new Boolean();

See the difference below. Because new Boolean() is an instance of boolean type and is an object, it returns true.

var a = new Boolean(false); // Not recommended
alert(a); // true
var b = false;
alert(a); // false

String type

The usage method is new String();

var a = new String("hello"); // "hello"

 

Number type

The usage method is new Number();

var a = new Number(1); // Not recommended
var b = 1; 
alert(typeof a); // "object",Number Object is Number Type, so return"object"
alert(typeof b); // "number",b Is a basic type, so it returns"number"
alert( a instanceof Number); // true
alert( b instanceof Number); // false

 

9. Single built-in object

9.1 Global objects

URI encoding method

The encodeURI() and encodeURIComponent() methods can encode the URI (replace all invalid characters with special UTF8 encoding) so that it can be sent to the browser for easy understanding.

encodeURI()	code	Mainly used for the entire URI. Special characters that are themselves URIs are not encoded.
decodeURI()	decode	Decode the URI encoded by encodeURI()
encodeURIComponent()	code	It is mainly used for a segment in the URI. Encode any non-standard characters found.
decodeURIComponent()	decode	Decode the URI encoded by decodeURIComponent()

Example:

eval() method

The eval() method, like the ES parser, accepts only one parameter.

eval("var ms = 'hello world';")
alert(ms); // "hello world"

 

window object

 

9.2 Math object

Math objects provide calculation capabilities. List some common methods:

floor(x)	Rounds x down.
max(x,y,z,...,n)	Returns x, y, Z, The highest value in n.
min(x,y,z,...,n)	Returns x, y, Z, The lowest value in n.
random()	Returns a random number between 0 and 1.
round(x)	rounding.

 

 

 

 

  

Reference article:

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Guide/Regular_Expressions