Common Objects in Js

Dater Date object

// New object
var date = new Date();

// Days of the week
date.getDay();

// Days of the month
date.getDate();

// Return month 0-11
date.getMonth();

// Return to the difference between 1900 and the present
date.getYear();

// What year is it now to return?
date.getFullYear();

// Return to local time
date.toLocaleString();

Mathematical function object

// Getting the Random Range of Random Numbers is [0-1]
var ran = Math.random() * 1000;

// Rounding up
Math.ceil()

// Rounding down
Math.floor()

// Get 4-digit authentication code
Math.random() * 9000 + 1000;

Sting object

// Get the content of subscript 2
str.charAt(2);

// Get subscript
str.indexOf("a");

// String interception
str.substr(2,4);
str.substr(1);
str.substring(2,4);

// Segmentation by specified characters
str.split("-");

Global object

Global objects can be used directly

// Execute strings as js code
var b = "var c = 1 + 1";
eval(b);

// Check whether a value is a number
isNaN("123a");

Array objects

Array declaration

Method 1:

var arr1 = new Array();

Mode two:

var arr2 = new Array(2); // 2 is the length of the array. At this point, the output array will show that the two elements of the array are empty.

Mode three:

var arr3= new Array(1, "Hello", new String(), true);

Mode four

// This is the same usage as java
var arr4 = ["jssd", 123, new Date(), false];

Arrays need to be used

The subscripts of arrays in js can be discontinuous and empty if there is no value

function demo() {
    var arr=[];
    arr[0] = "Fur"; // Automatic expansion to length = 1;
    arr[1] = true; // Automatic expansion to length=2
    arr[6] = 12; // Automatically expand to length=7, empty is the array that has no previous assignment
}

Expansion of arrays

function demo() {
    var arr=["jssd", 123, new Date(), false];
    // Expanding array, empty unassigned
    arr.length = 10;
    // Reducing the excess of the array will be discarded
    arr.length = 2;
}

Traversal of arrays

One way

function demo() {
    var arr =  ["jssd", 123, new Date(), "Test it."];
    for (var i = 0; i < arr1.length; i++) {
        alert(arr1[i]);
    }
}

Mode two

 var arr =  ["jssd", 123, new Date(), "Test it."];
    for (let x in arr1) { // x is the subscript of an array
        console.log(arr[x]);
    }

Mode three

var arr = ["jssd", 123, new Date(), "Test it."]
for (let x of arr1) {
    console.log(x);
}

Common methods in arrays

// Add one or more elements to the end of the array and return the length of the result
arr.push("We");

// Delete and return the last element of the array
arr.pop();

//Add one or more elements to the beginning of the array and return a new length
arr.unshift("Hello");

// A first element
arr.shift();

// splice function
arr.splice(1, 2); // Delete from subscript 1, delete 2 elements
arr.splice(1, 0, "you", "good"); // Starting from subscript 1, delete 0 elements and add two elements. The function of adding elements at specified positions is realized.