Introduction to core JavaScript data types and variables

Source: Internet
Author: User

Any programming language has its own language core. Like many programming languages, JavaScript also has its own language core. The core part of a language is generally called the most basic part of JavaScript. The saying goes that everything is difficult at the beginning. Learning JavaScript is also a truth. There is also an old saying that a good start is half the success. The core part of understanding and learning JavaScript is a very good start for JavaScript learning.

The following code describes the JavaScript data types and variables:
Copy codeThe Code is as follows:
// In the script tag, all the content following // is called annotation.
// Comments are generally used to explain a piece of JavaScript, so that other Web Front-end development engineers can be more clear when reading this code.

// A variable is a symbolic name of a value. Through the names of these variables, we can also roughly know what the variable is for and what type it belongs.
// It is easy to differentiate variables. There is "var" before the variable, that is, the variable is declared through "var.

Var m; // declare a variable m

// Generally, the value to be declared is assigned to the variable with an equal sign.
Var m = 10; // The current variable m is equal to 10

M // use the variable m to obtain the declared value

// Alert (m) // use the alert () function to pop up the m value in the browser

// JavaScript data type: Boolean value, number, String, underfind, function, array, Object

Var n = 1; // number
N = 0.01 // integer and real number are both numerical

Var s = "hello"; // a string consisting of text in double quotation marks
S = 'guo'; // a string consisting of text in single quotes

Var B = false; // The Boolean value of the Error
B = true; // The correct Boolean value.

Var z = null; // a null value, which is a special type and is an object after typeof

Var u; // underfind

Var j = {// an object representing json
Li: 3, // The value of the attribute "li" is 3
Meng: 4 // The value of the attribute "meng" is 4
}

J ["li"] // access the value in json through []
J. li // access the value in json through.
J. long = 5 // create a new property by assigning values.
J. ai = {// you can create a new json object by assigning values.
Xin: 33
}
J. kong = {}// {} indicates an empty object. It has no attribute.

J. ai. xin // access attributes in the new json through.

Var a = [2, 3] // an object representing an array

A [0] // [] in the array represents the position where the number in [] represents, and the array starts from 0, so a [0] is the first element of the array.
A. length // length indicates the number of array a, 3
A [a. length-1] // represents the last element in the array
A [9] = 2; // use the value assignment method to add a new element.

// If a = [], it indicates that the elements in the array are zero, and a. length = 0

A [0] = {
Li: 333
}

// Arrays can be included in json, and arrays can also contain json

// Alert (a [0] ["li"])

In the above code, you can use "[]", "{}", ". "defines the object, you can also use" [] "," {} ",". to change the content of an array or object. You can also use "[]", "{}", and "." to read some data in the object. The following code is related to operators:
Copy codeThe Code is as follows:
// You can use the budget operator in JavaScript to calculate two numbers and generate new values.
// The following are common budget characters, such as "+", "-", "*", and ","-","*","/"

// 1. Operator Introduction
10 + 10 // addition, 20
10*10 // multiplication, 100
10-10 // subtraction, 0
10/10 // Division, 1

Var j = {// an object representing json
Li: 3, // The value of the attribute "li" is 3
Meng: 4 // The value of the attribute "meng" is 4
}

J ["li"]-j. meng // The property li in json j minus the property meng in json j. The result is-1.

"10" + "10" // addition can be used to concatenate strings. The result is 100.

// 2. Some short operators are defined in JavaScript.

Var num = 0 // define a number

Num ++; // indicates auto-increment, num = num + 1;
Num --; // indicates auto-subtraction, which is equivalent to num = num-1;
Num + = 2; // indicates auto-increment 2, which is equivalent to num = num + 2;
Num * = 8; // indicates 8, which is equivalent to num = num * 8;

// 3. Operator judgment

Var a = 1, B = 2; // an equal sign represents replication, and two variables are separated by ",", indicating simultaneous declaration.

A = B; // The result is false. Does it mean that a and B are equal?
A! = B; // is the result true, which means that a and B are not equal?
A <B; // does the result mean that a is smaller than B?
A <= B; // if the result is true, does a mean that a is less than or equal to B?
A> B; // if the result is false, does a mean that a is greater than B?
A> = B; // if the result is false, does a mean that a is greater than or equal to B?
"Two" = "three"; // true "tw" the index in the alphabet is greater than "th"
False> (a> B) // The result is true, which means comparison between false and false.

// 4. logical operators

(A = 2) & (B = 3) // The result is true. Is a equal to 2 and B equal to 3. & Meaning of and
A> 2 | B> 2 // The result is true. The first value is false, and the second value is true. | indicates or.
! (A = B) // The result is true .! Is reverse.

In the operator, only calculating a value does not affect any operation, but does not change the running status of the program. The statement does not contain a value, but changes the running status. The statement changes the running status, so it is followed by a plus sign.

Each function has its own name. You can call and execute a function by name, which can be defined once and called multiple times. The following is a simple example of a function.
Copy codeThe Code is as follows:
// 1. A function is a JavaScript code with parameters. It can be defined at a time, called multiple times, or with parameters.

Var a = 3; // declare a variable a with a value of 3;

Function fn1 (n) {// a function whose parameter is n named fn1
Return n + 1; // return a value greater than the input value.
}

Fn1 (a) // The result is 4. Because the declared value of a is 3, when calling a function, execute a + 1, that is, 3 + 1.

Var fOne = function (m) {// The function is also a data type, so you can assign a variable to a function.
Return m * m; // return a value for parameter * Calculation
}

FOne (a) // The result is 9

// 2. assign a function to the attributes of a variable.

Var arr = []; // create a new array
Arr. push (, 3); // use the push () method to add elements to the arr array from the back.
Arr. reverse (); // use the reverse () method to reverse the order of elements in the array

Var points = [// declare a json array in an element
{A: 0, B: 0 },
{A: 1, B: 1}
]
Points. dist = function () {// define a method for calculating the distance between two points in the declared Array

Var p1 = this [0]; // use this to obtain the reference of the current array.
Var p2 = this [1]; // and assign two new variables
Var a = p2.a-p1.a; // distance on the X axis
Var B = p2. B-p1. B; // distance on the Y axis

Return Math. sqrt (a * a + B * B) // use sqrt () in Math () to calculate the square root of two points.

}

Alert (points. dist () // result 1.414

// 3. Control statement
// Conditional statements and cyclic statements are called control statements.

Function abs (m) {// returns the absolute value function.

If (m> = 0) {// if the comparison result is true
Return m; // return m
} Else {// If the comparison result is false
Return-m; // return-m
}

}

Function factorial (n) {// calculate the factorial function

Var num = 1; // declare a variable with a value of 1

While (n> 1) {// when the expression in () is true, execute the code in the loop {}

Num * = n; // equivalent to num = num * n
N --; // equivalent to n = n-1

}

Return num // return the factorial result

}

Factorial (4) // The result is 24

Function factorialFor (n) {// use the for loop to implement factorial
Var I, num = 1; // declare variable I, and declare the variable num with a value of 1

For (I = 2; I <= n; I ++) {// cycles I from 2 to n
Num * = I; // The cyclic body. It can be omitted if there is only one sentence in the cyclic body {}
}

Return num; // return the calculated factorial table

}

FactorialFor (5)

From the function example, whether it is a while loop or a for loop, whether it is a judgment statement or a loop statement, it can be regarded as a control statement. Use certain conditions to control what will happen.

After introducing the functions, we will briefly introduce object-oriented functions.
Copy codeThe Code is as follows:
// Define a constructor, that is, first create an initialized object

Function Point (x, y) {// The first letter of the constructor name must be capitalized.
This. x = x; // this indicates the initialization object.
This. y = y; // Save the function parameters to the attributes of the initialization object.
} // The constructor does not need to return anything.

// Use the new Keyword and constructor to create a new object
Var p = new Point (); // create a Point with the plane coordinate ()

// Assign a value to the constructor prototype to add a method to the newly created object of the Point.
Point. prototype. r = function (){

Return Math. sqrt (
This. x * this. x + this. y * this. y
); // Use the sqrt () method in Math to calculate the square root. This refers to the object that calls the method.

}

P. r () // The result is 1.414.

The above example shows how to define a vertex with the square root method. JavaScript Object-oriented has some differences with other programming languages. What is the specific difference? We can only continue to study it.

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