This points to related issues-JavaScript (definitely worth understanding)

code 

        <script>
            var num = 100;

            var obj = {
                num: 200,
                inner: {
                    num: 300,
                    print: function () {
                        console.log(this.num);
                    }
                }
            }

            obj.inner.print(); //300

            var func = obj.inner.print;
            func();  //100

            (obj.inner.print)(); //300

            (obj.inner.print = obj.inner.print)(); //100
        </script>

question

  • what"s the difference between the first and the third?
  • what"s the difference between the third and the fourth?
This javascript
Mar.02,2021

1. There is no difference between the first one and the third one. The this in obj.inner.print (), points to obj.inner.num

.

2. The fourth

first of all, you need to know that the copy operation returns the assigned value 

var a;
console.log(a = 5); //5

so the result of (obj.inner.print = obj.inner.print) is a function with the content 

function () {
    console.log(this.num);
}

run this function globally, and the this in it points to window, so the result of (obj.inner.print = obj.inner.print) (); ) is

. 
var num = 100;
function () {
    console.log(this.num);
}()
// 100

looking at the answer, the main confusion lies in the third formula. In fact, it is easy to understand, do not be involved in the second and fourth.

the second is an assignment, and the assigned func is just a function reference, which loses the context information of the function, so it ends up running

in the global context.

Why not the fourth one? The reason is that the side effect of the assignment expression : the assignment expression returns the value to the right of the equal sign! So the result of the entire assignment expression is a function that also loses its context. So the result is the same as 2. If you replace the equal sign with a comma, you will get the same result, because the comma expression has the same side effect.

what's the problem with looking at 3pr 3 again? No problem! Because include! Number! no! Yes! Deputy! Do it! Use ! You said it would return an expression, an expression is an expression, and whatever, alone looks at the parenthesis of the first formula and the part before it is also an expression, isn't it?

post other people's blog answers . I think starfish

is the "mentioned on the home page of banner to thoroughly understand the this mechanism in JS

explain it in the way" you don't understand js ":
first, third, implicit binding. This binds to inner object
two and four are default bindings, and this binds to window object

Previous: Will mongodb $set produce dirty data when updating different fields of the same document?

Next: Mini Program uses canvas to draw pictures and uses developer tools to achieve editing, but it is not possible to upload them online. Why?

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
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3699e-2c02d.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3699e-2c02d.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?