Niuke JS wrong question knowledge point record

plus + The definition of

about “+” Come on , There are two meanings ： The first meaning is string splicing , The second meaning is addition in addition and subtraction .
1, If one of the operands is a string , Other values will be converted to strings ;
2, Other situations , Operands are converted into numbers to perform addition operations .
And for “ - ” Come on , There is only one meaning , It's subtraction , Naturally, it will not be converted into a string .

web Page lifecycle

The loading sequence of general browsers is as follows ：

• script executed
• readyState : interactive
• DOMContentLoaded
• image onload
• iframe onload
• readyState : complete
• window onload

summary ：

• synchronous JavaScript Script executes first , It precedes DOMContentLoaded Event to perform .

• When DOM When ready ,DOMContentLoaded The incident happened in document Trigger on . Can be used at this stage JavaScript To operate DOM Elements .

    All scripts have been executed , Except for those external uses of asynchrony （async） Or delay （defer） Loaded script 
    Images and other resources may still be loading .
  

• window Upper onload event , Trigger after all resources are loaded on the page . Most of the operations we should put in DOMContentLoaded In the event , Instead of putting it in window.onload In the implementation of , Because the usual operations don't have to wait until the end .

• window Upper beforeunload event , This event occurs when the user is ready to leave the page , stay unload Triggered before the event . If beforeunload Returns a string , The browser will give a prompt whether the user really wants to leave .

• window Upper unload event , This event will be triggered when the user finally leaves . stay unload In the event handler of , We can only do simple things , It does not involve delaying or asking the user . Because of this limitation , It's rarely used .

• document.readyState Indicates the current status of the document , Can be in readystatechange Event to track changes in document status .

   loading –  Document loading .
   interactive – document Triggered when the resolution has been completed , Almost DOMContentLoaded At the same time , But in DOMContentLoaded Triggered before the event .
   complete –  Triggered when the document and resources are loaded , Almost window.onload At the same time , But in onload Triggered before the event .
  

Factors leading to backflow

1. Resize window , Browser window size changed ——resize Event time
2. Element size change —— Margin 、 fill 、 Frame 、 Width and height ; Change font size
3. Page rendering initialization
4. Element position changes
5. Style sheet changes
6. Element content changes , Especially the input control
7. CSS Pseudo class activation
8. DOM operation , Add or remove visible DOM Elements
9. offsetWidth, width, clientWidth, scrollTop/scrollHeight The calculation of ,
   Will cause the browser to gradually return to the queue Flush, Perform backflow immediately .
10. Content change —— For example, the width and height of the calculation value caused by the change of the text or the size of the picture