A function of js, the basic problem of variables. Come in and have a look.

function Foo(){
    bar=function(){console.log(1)};
    return this;
}

Foo.bar=function(){console.log(2)}
Foo.prototype.bar=function(){console.log(3)};
var bar = function(){console.log(4)}
function bar(){
    console.log(5);
}



new bar();
Foo.bar();
Foo().bar();
bar();
new Foo().bar()
//4 2 1 1 3

in this way, the function and variables will be improved after the browser is compiled, what will happen in the end? why will new bar print 4 console.log (5)? is it permanently overwritten?

Javascript front-end
Apr.19,2021

here is mainly function declaration promotion, function expression is not promoted. The promoted code is 

var bar;
function Foo() {
    bar = function() { console.log(1) };
    return this;
}

function bar() {
    console.log(5);
}

Foo.bar = function() { console.log(2) }
Foo.prototype.bar = function() { console.log(3) };
bar = function() { console.log(4) }


new bar();//4
Foo.bar();//2  Foobar bar
Foo().bar();// 1
bar();// 1
new Foo().bar()//3
function bar(){
    console.log(5);
}

will improve the function, which is equal to
at the top and then overwritten by var bar = function () {}

.

1. Declare promotion, so var bar and the following function definition are both promoted
2. Execution continues after promotion, and bar is assigned to the function body of 4, so 5 is overwritten

A little bit:
the first step, the js interpreter parses and compiles the js code, and then you know the function declaration promotion step.
this is the source code: 

new Foo().bar();
//  FoonewFoobarnew  Foo
// Foo bar barFoobarPrototypebar  3

Previous: How to introduce sass into vuepress

Next: Is there a mistake in the book "the authoritative Guide to let" about the use of let in js? At least not up to current standards?

css mysql arrays josn react html typescript webpack npm sass R objective-c .net sql-server jquery python-3.x angularjs django angular excel regex iphone ajax linux xml pandas vba spring database wordpress string wpf xcode windows bash postgresql oracle multithreading eclipse list firebase algorithm macos forms image scala visual-studio azure bootstrap spring-boot react-native python-2.7 docker performance function winforms matlab powershell apache dataframe api sqlite numpy rest shell selenium flutter dart maven loops qt swing android-studio csv express file class tensorflow sorting codeigniter perl
© 2021 CodesHelper
Advertising Contact us About us
Menu