About function A () {} and var a=function () {}

Cases:

a (); B (); function A () {Console.log (111);} var b = function () {Console.log (222);}

Run Discovery 111, B is not a function;

This is because function a () {} is pre-compiled before the code executes.

What is the pre-compilation of JS?

JS will explain both the Var and function declaration keywords in advance, and the assignment of variables is done when the code executes.

That is: var B = function () {} at the pre-compilation stage, the equivalent of Var b, and at execution time, because from top to bottom execution, so when executed to B (), the browser only know that Var B, and has not been assigned (undefined), so reported B is not a functio N error.

About function A () {} and var a=function () {}