How to understand JavaScript pre-parsing and object
This article shows you how to understand JavaScript pre-parsing and objects. The content is concise and easy to understand, which will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.
1. Pre-analysis 1. Variable pre-analysis and function pre-analysis.
The JavaScript code is executed by the JavaScript parser in the browser. The JavaScript parser runs JavaScript code in two steps: pre-parsing and code execution.
Pre-parsing: in the current scope, the browser will declare or define variables with var and function declarations in memory by default before the JS code is executed.
Code execution: executes the JS statement from top to bottom.
Pre-parsing occurs only on variables and function defined by var. Learning pre-parsing can let us know why the value of the access variable is undefined before the variable is declared, and why the function can be called before the function is declared. Pre-parsing is also called variable and function promotion.
1. Variable pre-analysis
Variable pre-parsing: the declaration of the variable is promoted to the top of the current scope, and the assignment of the variable is not promoted.
For example:
/ * first parse the var variable num, then execute the console output and finally assign 10 to num*/console.log (num); / / what is the result? Var num = 10; /? 2. Function pre-analysis.
Function preparsing: the declaration of the function is promoted to the top of the current scope, but the function is not called.
/ * parse the definition of the fn function first and then execute the console statement * / console.log ("1 / 2 / 3 / 1 / 2 / 3 / 1 / 2 / 3 / 1 / 3 / 1 / 3 / 1 / 3 / 1 / 3 / 1 / 3 / 1 / 3 / 1 / 3 / 1 / 2 / 1 / 3 / 1 / 3 / 1 / 2 / 1 / 3 / 1 / 2 / 3 / 1 / 3 / 1 / 3 / 1 / 2 / 3 / 1 / 2 / 3 / 1 / 3 / 1 / 1 / 2 / 1 / 2 / 3 / 1 / 3 / 1 / 3 / 1 / 2 / 3 / 1 / 2 / 3 / 1 / 1 / 2 / 3 / 1 / 3 / 1 / 2 / 3 / 1 / 3 / 1 / 2 / 3 / 1 / 2 / 3 / 1 / 3 / 1 / 2 / 3 / 1 / 2 / 3 /
