Small knowledge, big challenge! This article is participating in the creation activity of “Essential Tips for Programmers”.

JS

What is it, it, it?

Js is one of the most popular scripting languages on the Internet

  • It is a lightweight programming language
  • It is a weakly typed language
  • It is programming code that can be inserted into an HTML page
  • It can be executed by all modern browsers
  • It’s easy to learn ！！！！！

It can do

Js is the best in the world (sure), so what can we do with it

Hold up a few chestnuts

  • When we log in, JS will make some judgments about our input data
  • Some of the beautiful animation effects in the web page can also be realized with JS
  • , etc.

The most basic debugging method

As a qualified programmer, we should first learn to debug our own programs,

To debug and see what went wrong, the most common thing we front-end programmers use is printing, right

var a = 'I love Grey Wolf';
console.log(a);
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Begin formal study

Where can we put the JS code

We’ve got HTML and CSS so where can we put JS?

  1. The inline type
<input onclick="Alert (' Rui Rui women's Dress ')">
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  1. embedded
<script>
     alert('js');
</script>
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  1. External introduction
<script src="./index.js"></script>
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Our old friend, the variable

Everyone has learned c language, so variables also have a certain understanding, but in JS variables are loosely typed, can save any value

Uninitialized variables hold a total of special values -undefined

Naming conventions for variables

Js variables have naming conventions

  • Variables must start with a letter
  • Variable names are case-sensitive (A and A are different variables)
let a;
let 1a;
let A;
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Declare a variable

As you can see above, there are three ways to declare a variable. The main difference is the scope of the variable

  1. Var:
  2. Let:
  3. const:

The data type

  1. Dynamic type

Dynamic typing is when the same variable can be used as different types

int a;
float c;
char a;
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var a = 1;
var b = "1"
var c = "123"
typeof(a)
typeof(b)
typeof(c)
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  1. string

A string is a variable that stores characters.

A string can be any text in quotes, either single or double quotes:

var carname="my name id cxy";
var carname='my name id cxy';
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Strings can also be concatenated that is, two strings can be concatenated into one string, 1+1=1

var a = "1"
var b = "2"
console.log(a+b)
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Of course, we can concatenate variables in string concatenation to solve the requirement that some content should change with data

var a = 20
console.log('cxy'+a+', ')
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There is no difference between single and double quotation marks in syntax, but only in specification. 3. Digital

Js has only one numeric type, which is very different from C

var a = 100;
typeof(a);
var b = 3.14;
typeof(b);
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  1. Boolean

Booleans have only true and false values

Arrays are something that we’re going to use a lot in our programs

var array = new Array(a); array[0] = 1;
array[1] = "qinli"
console.log(array);
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  1. Undefined

In JavaScript, a variable has no value; its value is undefined. Typeof also returns undefined.

var a;
console.log(a)
/ / output is undefined
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  1. Object 👩 🦰 🧑

Does everyone know what the object is 😛

In fact, it’s similar to the structure of C for example, if I declare a student’s structure, give it some properties, like name, age, major, etc. So we declare an object in JS, give it some properties like name, age, gender.

var person={
    name : "John".id :  5566
};
console.log(person.name)
console.log(person["name"])
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  1. Null

NULL is a special variable

typeof(null)
// The output is object
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This is because null represents a pointer to an empty object. The difference from undefined is that it is null, and undefined is undefined

Our right-hand man — functions

Function declarations and function expressions

  1. Function declaration
function myname(){
     var name = "cxy"
     return name;
}
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  1. Functional expression
var myname = function(){
     var name = "cxy"
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What’s the difference?

Probably also read the difference between it

The function declaration will be read by the parser first, and the function expression will wait “a long, long time” to be parsed, and an error will be reported if called.