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If we have embedded in the HTML <script> JS coding </script> as JS block of code (in fact, JS does not have a block of code meaning, if really want to think of code block that also should be divided by function block), Here is the JS Code execution order:
Here reproduced JS execution sequence summary, please refer to http://www.3lian.com/edu/2014/04-07/139469.html
Step 1. Read in the first block of code <script> JS Coding </script>, in the order in which they are declared in HTML.
Step 2. Do grammar analysis, parsing the basic syntax format, error is reported grammatical errors (such as parentheses mismatch, etc.), and jump to step5.
Step 3. The var variable and the function definition are "precompiled" (never error, because only the correct declaration is parsed), that is, these var variables and function will be first preprocessed, that is, stored in memory, but the variable is only declared, and is not assigned a value, Any block of code underneath it can execute calling functions and variables.
Step 4. Execute code snippet, error (such as variable undefined), after error, if there is no catch then this code execution end, the variable after the error is not assigned value
Step 5. If there is a next code snippet <script> JS coding </script>, read the next code block and repeat Step2.
step6. End.
Order of execution based on HTML document flow
The JS code that needs to be executed before the page element is rendered should be placed in the <script> code block in the <body> front.
Instead, after the page element is loaded, JS is placed behind the </body> element,
The function that is registered with the OnLoad event of the body tag is executed at the end. Also keep in mind that Window.onload is the OnLoad event registered on the body, so if we
The body declares window.onload = function () {}; It will not work because it is overwritten by the onload in the body, however, if we
Window.onload The block of code that is placed after the body, then he overwrites the OnLoad event registered in the body, which is relative to $ (document). Ready (function ()
{}) to be executed later, jquery is run after the DOM tree has been loaded, that is, the label is in place, and it is not as window.onload requires all resources to be loaded.
You want to get the height or width of an element, as long as the element is loaded, but the display must not be none
<! doctype html>
JS statement execution order in Javascript
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