Lecture 1 of 'Advanced Programming with JavaScript' -- Basic Concepts

I have read the book "JavaScript Advanced Programming" several times before, but I have never read it completely. From now on, I will read it completely and record notes and my own thoughts on each chapter. Come on!

Since the content of the first three chapters is relatively simple, I will put them into one chapter to record the learning process.

1. Basic concepts

1. The relationship between JavaScript and ECMAScript

It can be understood that in addition to ECMAScript, JavaScript contains Also includes DOM (Document Object Model), BOM (Browser Object Model)

2, <script> tag</script>

• Attributes: async defer charset type src

The difference between defer and async: (only applicable to external script files)

Without async or defer, the <script> tags are parsed in sequence. The browser will load and execute the specified script immediately. "Immediately" refers to before rendering the document element under the <code>script tag. That is to say, it does not wait for subsequent loading of document elements. Load and execute. </script>

With async, the process of loading and rendering subsequent document elements will be carried out in parallel with the loading and execution of js (asynchronously);

With defer, the process of loading subsequent document elements will be parallel with the loading of js Proceed in parallel (asynchronous), but the execution of js must be completed after all elements are parsed and before the DOMContentLoaded event is triggered

The blue line represents network reading, and the red line represents Execution time, both are for scripts; the green line represents HTML parsing.

Click for detailed analysis

• ## Tag position: can be placed behind the content in the or element

• Document mode: (can be divided into mixed mode and standard mode)

The difference between browser mode and document mode in IE:

Browser mode: It affects the browser version and IE’s conditional comments.

Document mode: It affects IE’s layout engine and DOM rendering.

Click for detailed analysis

3. Data type

5 simple data types: String, Number, Boolean, Null, Undefined 1 complex data type: Object

The typeof operator detects the variable Data type, return string

Note: typeof null returns 'object', null is considered an empty object reference

When the variable is not initialized or declared, typeof All return undefined

　----- Boolean type

#Data typeConvert to true value Value converted to falseBooleantruefalseStringNon-empty stringEmpty stringNumberAny non-zero numeric value (including infinity)0 and NaNObjectAny objectnullUndefinedNot applicable undefined

　-----Number type

　The calculation of floating point numbers in js is not accurate, use it with caution~

　isFinite() Determines whether a number is within the minimum sum The largest number between ​

　NaN Non-numeric value, not equal to anything, including itself ​ ​ ​ isNaN() ​

　 Non-numeric value converted to numeric value: Number() Number('') 0 Number( null) 0

　　　　 parseInt() parseFloat() If the first character is not a numeric character or a negative sign, then NaN is returned parseInt('') NaN parseInt(null) NaN

--- -- String type

Method to convert string: toString() Except for null and undefined, this method is available. String() can convert any type of value into a string. Plus operator +''

　 ----- Object type

Each instance of Object has the following properties and methods

Constructor: Saves the function used to create the current object

hasOwnProperty(propertyName): Whether the property exists in the current object instance

isPrototypeOf(object): Whether the incoming object is the prototype of another object

PropertyIsEnumerable(propertyName): Whether the property can be used for-in statement to enumerate

　　toLocaleString()

　　toString(): Returns the string representation of the object

　　valueOf(): Returns the string, numeric or value of the object Boolean value representation

4, operator

　var age = 10; var newAge = ++age; console.log(age); //11 console.log(newAge) ; //11 ++age is added first and then assigned, that is, first age+1, and then assign the value to newAge

var age = 10; var newAge = age++; console.log( age); //11 console.log(newAge); //10 ++age is assigned first and then added, that is, assign the value to newAge first, then age+1

　----- Bitwise operator

Bitwise not (~) means the negative value of the operand is reduced by 1

Bitwise AND (&) returns 1 only when the corresponding bits of both values ​​are 1, any one If the bit is 0, the result is 0

Bitwise OR (|) returns 1 if one bit is 1

Bitwise XOR (^) The corresponding bits of the two values ​​only have Returns 1 only when there is a 1

Left shift (

Signed right shift (>>) fills the empty bits with the value of the sign bit

Unsigned right shift (>>>) fills the empty bits with 0

　----- Boolean operator

Logical NOT (!)

Logical AND (&&) short-circuit operator, that is, if the first operand evaluates to false, then The second operand will not be evaluated eg:console.log(1 && 2 && 3); //3

Logical OR (||) short-circuit operator, that is, the If the evaluation result of one operand is true, the second operand will not be evaluated eg:console.log(1 || 2 || 3); //1

　----- Equality operator

　null >= 0 //true null == 0 //false Reason: The relational operator will be converted into a numerical value when compared, but the equality operator will not Will

　

null == undefined //true null == 0 //false undefined == 0 //false

　= =Only compare values ​​===Compare both values ​​and types

A question asked in the previous interview: var a = 1, b = 1; var c = {a: 1}, d = {a:1}; console.log(a == b); console.log(a === b); console.log(c == d); console.log(c === d); //true true false false

When the comma expression is used for assignment, it will always return the last item in the expression eg:var num=(1,2,3 ); console.log(num); //3

5. Statement

do{} while () statement while() {} statement switch statement

For-in statement: can be used to enumerate the properties of an object. The order of returning properties may vary depending on the browser

break statement: exit the loop and execute the statement after the loop continue: jump out of this loop, Execute the next loop

6, function

The arguments object can access the array of parameters. The value of the parameters can be obtained through arguments[0]..., and has the length attribute

Functions that do not specify a return value return an undefined value. This is why sometimes entering some commands in the chrome console will return undefined~

 

 

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