JavaScript Basics (must look) _ Basics

The language has been in contact with JavaScript for a long time, but it has never been systematically understood. Take advantage of the current graduate and some reason not to work the situation of the system to understand the language, but also want to use such language to develop the habit of blogging, because I think it is a very sacred and glorious thing for programmers.

1.1 Background

It is believed that many beginners have forgotten or confused the official name of JavaScript: ECMAScript. June 17, 2015, ECMAScript 6 issued an official version, that is, ECMAScript 2015.

1.2 Grammar

General syntax ellipsis

Emphasis is placed on:

1. Original values and objects: The original values include Boolean values, numbers, strings, null, and undefined. The other values are objects. The main difference between the two is the way they compare: Each object has a unique identity and is equal to itself.

var obj1={};
var obj2={};
Alert (obj1 = = obj2);

False

alert (OBJ1===OBJ1);

True

var prim1=123;
var prim2=123;
alert (PRIM1===PRIM2);

True

2. Use typeof and instanceof to classify values.

typeof

Number of operands	Results
Undefined	' Undefined '
Null	Object
Boolean value	Boolean
Digital	Number
String	String
Function	function
All other general values	Object
Value created by the engine	The JS engine can be allowed to create some values, and typeof results can return any string

3. Boolean value:

False value: Undefined,null,false,-0,nan, "

Binary logical operators: The two-yuan logical operator in JavaScript is shorted. If the first shipment is sufficient to determine the result, then the second operation will not be evaluated. With (&&): If the first shipment count is a false value, return it. or (| | ): If the first count is the truth, return it.

4.IIFE:

Introduce a new scope. Effect: eliminates unintended sharing of closures (functions and variables in the surrounding scopes to which they are connected).

Cases:

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 line that is marked (1) returns the current value of I, not the value of the function when it was created. After the loop ends, the value of I is 5, so all the functions in the array return this value. If you want to mark (1) This line 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});     
 } ()
) ; 
}

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