Basic JavaScript (mandatory) and javascript

Basic JavaScript (mandatory) and javascript

It takes a long time to get started with the JavaScript language, but it has never been understood by the system. While I have just graduated and have no intention of working for some reasons, I want to use this language to develop my blog writing habits, because I think this is a sacred and glorious thing for programmers.

1.1 Background

I believe that many beginners forget or confuse the official name of JavaScript: ECMAScript. On March 6, June 17, 2015, ECMAScript 6 was officially released, namely ECMAScript 2015.

1.2 syntax

General syntax omitted

Emphasis:

1. Original values and objects: the original values include boolean values, numbers, strings, null values, and undefined values. All other values are objects. The main difference between the two lies in their comparison method: each object has a unique identifier and is only equal to itself.

var obj1={};var obj2={};alert(obj1 === obj2);//falsealert(obj1===obj1);//truevar prim1=123;var prim2=123;alert(prim1===prim2);//true

2. Use typeof and instanceof pair value classification.

Typeof

Operands	Result
Undefined	'Undefined'
Null	Object
Boolean Value	Boolean
Number	Number
String	String
Function	Function
All other common values	Object
Value created by the engine	The JS engine can be used to create some values, and the typeof results can return any string.

3. boolean value:

False value: undefined, null, false,-0, NaN ,''

Binary logical operators: Binary logical operators in JavaScript are short-circuited. If the first operation is sufficient to determine the result, the second operation is not evaluated. And (&): If the first operation number is a false value, return it. Or (|): If the first operation is a true value, return it.

4. IIFE:

Introduce a new scope. Purpose: Remove unintentional sharing caused by closures (functions and variables in the surrounding scopes connected to them.

Example:

var result=[];for(var i=0;i<5;i++){result.push(function(){return i;});//（1）}console.log(result[1]()); //5  (not 1)console.log(result[3]()); //5  (not 3)

The return value of this line marked as (1) is always the current value of I, rather than the value when the function is created. After the loop ends, the value of I is 5, so all functions in the array return this value. If you want to mark (1) this row of functions to get a snapshot of the current I value, you can use IIFE.

for(var i=0;i<5;i++){ (function (){  var i2=i;   result.push(function(){return i2});      }()) ; }

This is part of the knowledge that you did not pay attention to or did not know during the sorting process. It is written here to supplement the knowledge points.

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