JS data type

The data type

Basic data types: Number, String, Boolean, NULL, undefind

Reference data types: function, Object, Arrary

Difference between NULL and undefind

null

Null indicates that the object is ready to be saved without actually saving its value. Logically, NULL represents an empty object pointer. Null is an object (empty object, with no properties or methods)

typeof null //Object

Typical uses of null:

As arguments to a function, indicating that the function’s arguments are not objects
As the end of the object prototype chain

undefined

Undefined: indicates a nonexistent value. Undefined is a primitive value that means “nothing” or something that means “missing value”, which means there should be a value, but it’s not defined yet, and undefined is returned when trying to get it.

typeof undefined //undefined

JS returns undefined:

A variable is declared, but not assigned, equal to undefined;

let a
console.log(a) //undefined
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If the parameter to be provided is not provided when a function is called, the parameter is undefined.

function fn(a){
    console.log(a) //undefined
}
fn()
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The object has no assigned attribute, the value of which is undefined;

let obj = {}
console.log(obj.name) //undefined
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If the function returns no value, undefined is returned by default

// Undefined is returned by default if the return value is not set
function fn(){}
let re = fn()
console.log(re) //undefined

// Set return, but do not explicitly return anything, return undefined
function fun(){
    return
}
let re2 = fun()
console.log(re2) //undefined
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Returns undefined when a variable is assigned to undefined

let a = undefined
console.log(a)  //undefined
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Note: It is never recommended to display the value of setting a variable to underFed, but it should be set to NULL if the variable used to store the object does not have a real store variable. But undefined is actually derived from null and returns true in the equality test.

Simple data types

The number type

Number is a numeric type. In JS, we can also use the built-in number constructor to create a numeric value. Literal creation is recommended.

<script>
    // Use the constructor to create a value
    let x = new Number('10');
    let y = new Number(5);

    // the literal creates a numeric value
    let z = 20;

    // Check if it belongs to the same constructor
    console.log(x.constructor === z.constructor);//true
</script>
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Common methods for numeric types:

ToFixed: Used to reserve several significant digits
ParseInt: Used to convert to a number type and take an integer
ParseFloat: Used to convert to a numeric type and take a floating point number

Type string

String is a string. In JS, we can also use the built-in string constructor to create a string value. Literal creation is recommended

<script>
    //1. Create a literal
    let str1 = 'Zhang Yi'

    //2. Create a constructor
    // Any string is equivalent to the instance object obtained by the new constructor
    /* String objects have properties: 1. length: the property used to get the string 2. Index value: example: STR [0] can get the first character */
    let str2 = new String('Zhang Yi')
</script>
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Common methods for string types:

Split: Splits a string into an array, adding ” converts each character to the value of the array
ToLocaleLowerCase: Convert letters to lowercase
ToUpperCase: converts letters toUpperCase
IndexOf: The index value of the first occurrence of a character. If it is found, the index value of the first occurrence is returned. If it is not found, -1 is returned
LastIndexOf: The index value of the last occurrence of the character, returns the last occurrence of the index if found, or -1 if not found
Trim: Removes whitespace at both ends of a string
Substr: Truncate a string, indicating where to start truncating how many

Syntax: string. Substr (start,length)

Note: If there is only one parameter, it is truncated from the start position to the end
Substring: Intercepts a string. The beginning character can be retrieved but the end character cannot

Syntax: string. Substring (start index value, end index value)

Note: If there is only one parameter, it is truncated from the start position to the end

Boolean type

Boolean types have two values: true and false. In JS we can create Boolean values through Boolean. Literals are recommended.

Default false values in JS: undefind, null, “empty string, number 0, number NaN, false

<script>
     // Use the constructor to create a Boolean type
      let locked = new Boolean('10');

      // literals create Boolean types
      let flag = true;

      // Check if it belongs to the same constructor
      console.log(locked.constructor === flag.constructor);//true
</script>
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Reference data type

The Object type

The object data type (object) can be created using the built-in object constructor to create a normal object. It is more recommended to create in literal form.

<script>
  // Create a normal object through the constructor
  let user = new Object({name: 'Ming'.age: 15});
  
  // Object syntax abbreviation
  let name = 'little red';
  // create in literal form
  let people = {
    // Equivalent to name: name
    name,
    Function () {}
    walk () {
      console.log('Everybody walks... '); }}</script>
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How common objects are stored in memory: The data of common objects is stored in heap memory, and the stack memory holds the address of common objects in heap memory. As shown in the figure:

Because the stack holds the address of heap memory, an assignment from an ordinary object assigns the address to another object. So whatever variable you change is going to change the data value of the other object pretty much. As shown in the figure:

Object method:

The object. assign static method creates a new Object
The object.keys static method gets all the properties of an Object
The object. values statement method gets the values of all attributes in an Object

Array type

The arrary type (Array) can be created using the built-in Array constructor to create an Array. It is more recommended to create in literal form.

<script>
  // The constructor creates an array
  let arr = new Array(5.7.8);

  // Create an array literally
  let list = ['html'.'css'.'javascript'];
</script>
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Arrays are stored in memory just like normal objects, as shown below:

Array assignment copies only the address in the stack, not the data in the heap, so the value of the other object will be substantially changed regardless of which variable is changed. As shown below:

Array method:Array method:

Pop deletes the last item of the array and returns
Push appends an element to the end of the array and returns the length of the new array
Splice Deletes or replaces. Syntax: array. splice(delete element subscript, delete number, replace the deleted element with the value written by the third argument) puts the deleted element into the new array and returns
Shift deletes the first item of the array and returns the deleted element
Unshift appends to the front of the array and returns the length of the new array
The instance method concat merges two arrays to generate a new array
The instance method reverse reverses the array
The instance method join array element is concatenated into a string. Array.join() and so on. The concatenation between two pairs of values in parentheses defaults to,
IndexOf looks for the first occurrence of an index in an array and returns the first index if it finds it, or -1 if it doesn’t
The lastIndexOf method looks for the last occurrence of the index in the array
The static method from a pseudo-array is converted to an array. Note: To convert a pseudo-array to a real array, you must have the Length attribute
The static method isArray checks whether a is an array
Find is used to find the value that meets the condition for the first time and returns the value, or undefind if no value is found
FindIndex is used to find the value that meets the condition for the first time and returns the index value. If no value is found, -1 is returned
Some searches for a value that satisfies the condition and stops as soon as it finds one, returning true for some and false for none
Every is used to find elements that meet the criteria, and returns true if they do, or false otherwise
Filter is used to filter the elements that meet the criteria, and all elements that meet the criteria are placed in a new array and returned
Map is used to iterate over each element of an array, performing a callback function that returns the function being played into a new array

Example method forEach is used to iterate over groups of numbers instead of for loops (🌟)

< script > let arr = [" zhang fei ", "zhaoyun", "guan yu", "d", "huang zhong", "the sable cicada"]. ForEach ((item, index, x) => {// The first argument represents each item of the array, the current element console.log(item); // The second argument represents the index of each element of the array, the index of the current item console.log(index); // The third argument represents the current array's own console.log(x); }); </script>Copy the code

Sort

Arr.sort ((a,b)=>{return a-b})

Arr.sort ((a,b)=>{return b-a})

JS data type

JS data type

The data type

Difference between NULL and undefind

null

undefined

Simple data types

The number type

Type string

Boolean type

Reference data type

The Object type

Array type

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