Commonly used knowledge points about JavaScript

Grammar

Statement expression

A sentence is a group of words that completely expresses a certain meaning, consisting of one or more It consists of phrases, and is connected between them by punctuation marks or connecting words.

Statements are equivalent to sentences, expressions are equivalent to phrases, and operators are equivalent to punctuation marks and connectives.

Expressions in JavaScript can return a result value.

　　　　　var a = 3 * 6;

　　　　　var b = a; = a; 　

Declaration statement, because it declares the statement

　　#The result value of the statement

The side effects of the expression

var a = 42;

var b = a++;

                                                                                                                                                                          ​ 

42 (Assign the value to

b),

Then add # to the value of

a

##1

；　　　　　++ When is in front, such as ++a, its side effects Incrementing (a) occurs before the expression returns the result and the side effect of a++ occurs after. 　　　　++a++ Will generate

ReferenceError

Error　　　　 var a = 42; 　　 　　 var b = (a++); The operator concatenates multiple independent expression statements into one statement: var a = 42,b; b = (a++,a) a ; // 43

　　　　　　b; // 43

　　　　 The delete operator is used to delete attributes in objects and elements in arrays. var obj = {

var obj = {

var obj = {

obj.a; // true

　　 　 　 obj.a;

　　　　　　　if(str) {

　　　　　　　　 //

Extract all vowel letters

　　　matches = str.match(/[aeiou]/g);

　　　　　　　　if(matches)　　　　　　　 return matches;

##　　　　vowels("Hello World" ); // ["e","o","o"]

Use the side effects of assignment statements to combine two if statements into one

function vomels (str){

var matches;

　　　　　　　　　　 // Extract all element letters

　　　　if(str && (matches = str.match(/[aeiou]/g))){

Return matches;

　　　Context rules

　　　 　　 1.

Braces

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　・　　　　　　　　　　　　　　　　　　credtrap ##Assume functionbar()

has been defined

var a = {

foo: bar() #　　　　　　　{　　　{}; / / "[object Object]"

　　　　　　　　{} + []; // 0

　　　　　Object destructuring

　　　　　　function getData() { b: "foo"

　　　　　　　}; }　　　　　　　　var { a , b } = getData(); can also be used as object destructuring of function named parameters to facilitate implicit assignment of object properties.

Function foo({a,b,c}){

//

No longer needs this:

// var a = obj.a, b = obj.b, c = obj.c　　　 console.log(a,b,c)

　　　　　　}

　　　　　foo({

o" [1 ,2,3]

Operator precedence

var a = 42;

var b = "foo" ;

　　　　　a && b; // "foo"

　　　　a || b; // 42

　　　Short circuit

　　　　For &&

and

||

, if the result can be obtained from the left operand, the right operand can be ignored

Operands, we call this phenomenon a short circuit.

　　　　　　a && b || c ? c || b ? a : c && b : a

　　　　 Because the && operator has a higher priority than ||, and || ## The priority of # is higher than? :.

　　　　　　(a && b || c) ? (c || b) ? a : (c && b) : a

　　　 Association

　　　　

Operator association is either left to right or right to left, depending on whether the combination starts from the left or from the right.

　　 　 var a = foo() && bar();

　　 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 through through [Execute first foo() ​ Follow the execution order from left to right

　　　　　　var a , b , c;

　　　　 a = b = c = 42; = ( c = 42))

var a = 42;

var b = "foo";

var c = false;

var d = a && b || c ? c || b ? a : c && b : a;

　　　　 d; // 42

　　　　((a && b) || c)

?

((c || b) ? a : (c && b)) : a

　　　　　 Now let’s execute

one by one　　　1. (a && b)

The result is "foo".

　　　　　 2."foo" || c The result is"foo ”.

　　　　　3,The first one? In "foo"

is true. 　　　　　　 4.(c || b) The result is "foo".

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　  　　　4.(c || , "foo"

is the true value.

　　　　　　 6.a The value of is 42

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　     The value of is 42

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　    The value of is

42

　　　　 Error

Errors that occur during the compilation phase are called early errors. Syntax errors are a type of early errors. Use try..catch

to capture

. Syntax errors are generally reported by browsers SyntaxError

Use variables in advance

Temporary dead zone

TDZ

let

Scope block

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

##　　　　　　　 a = 2; // ReferenceError

　　　　　　 let a;

　　　}

Function parameters

function foo(a = 42, b = a + 1){

console.log(a,b)

}

　　　 foo(); // 42 43

　　　 foo(undefined); // 42 43

　　 foo(5); // 5 6

　　 foo( void 0, 7); // 42 7

foo(null) // null 1 try.. finally The code in finally will always be try will be executed after, if there is catch , it will be executed after

###catch ######. ######

You can think of the code in finally as a callback function, which is always executed last.

Switch

You can think of it as a simplified version of if..else if.. else .

Switch (a) {

case 2:

# #Execute some code

　　　}

Mixed environment

JavaScript

Dynamically create

script, tags and add them to the

DOM

of the page.

var greeting = "Hello World";

var el = document.createElement("script"); el.text = "function foo() { alert(greeting); setTimeout(foo,1000)}";

Document.body.appendChild(el); If el.src If the value is set to URL

, you can use

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