Small knowledge of javascript functions

　　Functions in javascript are very different from functions in other languages, which makes people feel weird. Especially with or without parentheses and with or without new, the results are very different, and the this point also has unpredictable changes.

<script type="text/javascript">
	//A little knowledge about functions, wallimn
	function func(){  
	    this.id='func';  
	}  
	  
	var a = func;//Assign the function corresponding to func to a, this function is not executed. After that, a() is equivalent to func(), that is, a and func point to the same function body.  
	console.log(a)//The output is the function definition  
	console.log(a.id);//The output is undefined
	console.log(window.id);//The output is undefined, because only the function assignment operation is performed, and the function is not executed  
	var b = func();//Assign the execution result of the func function to b, and the func function does not return a value, then b is undefined. In this process, this in the func body refers to window. According to the output of window.id, it can be seen that  
	console.log(b)//The output is undefined  
	//console.log(b.id)//There will be an error in execution.
	console.log(window.id);//The output is func, when the function is called normally (without new), the internal this points to window  
	var c = new func();//It is equivalent to executing func as a constructor, returning an object and assigning it to c. At this point, c.id can be called. This in the function body points to the context formed after the function is executed  
	console.log(c);//The output is the created object, and the this pointer points to the function execution context.  
	console.log(c.id);//out as func  
	var d = new func ();  
	d.id="new func";  
	console.log(c.id);//The output is func  
	console.log(d.id);//The output is new func. Each time new, a separate context is obtained.  
</script>