The function declaration in JS exists "use first, then define"

First look at a section of JS code, which uses two ways to declare two functions, respectively, in different places call two functions:

1 <script> 2     ' use strict '; 3//     output Hello function 4     console.log (hello); 5     //define Hello function 6     function Hello () {7         alert ("Hello"); 8     } 9     //Output hi variable     console.log (HI);     //Assign a function to Hi     var hi = function () {         alert ("HI");     }15     //output variable hi     Console.log (HI); </script>

In JS, there are two ways to define a function, namely "function statement" and "expression".    -For the Hello function, I called it before I defined it, and I defined how the "function statement" used by the Hello function was defined. -For the HI function, I used the definition of "expression", which was also called before and after the definition. The results are as follows: we can see that functions defined in the way of "function statements" can be "used first, then defined". A function defined with an expression can only be "defined, then used".

The function declaration in JS exists "use first, then define"