javascript - I would like to ask you a question about function promotion in JS?

Question

The following code:

function a() {
    console.log('1')
}    

(function() {
    console.log(a);
    if(1) {
        function a() {
            console.log('2');
        }
    }
})()

After running, the output is undefined.

After removing the if condition, the output is the second a function

function a() {
    console.log('1')
}    

(function() {
    console.log(a);

    // if(1) {
        function a() {
            console.log(2);
        }
    // }
})()

Knowing that the function has been promoted, in the second piece of code, the second a function will be promoted to before the console.log(a) code, so the second a function is run and output.
But in the first piece of code, I don’t understand why undefined is output.

Answer 1

Conditional function declarations are processed in the same way as function expressions. Therefore, conditional function declarations lose the hoisting properties of function declarations.

Reference URL: /q/10...

Answer 2

When using the function keyword in an if else statement to declare a function, the promotion of variables is different in different browsers. It's just that the declaration of a variable is promoted here, and if else is removed, it becomes a pure function scope.

function a() {
    console.log('1')
}    

(function() {
    var a;
    console.log(a);
    if(1) {
        a = function a() {
            console.log('2');
        }
    }
})()

Answer 3

In your IIFE

 if(1) {
  a = function a() {
    console.log('2')
  }
} 

is a function expression, not a function declaration. When the if is removed, it is a function declaration. If is not removed, conosle.log(a), a represents the undefined variable a, refer to https://developer.mozilla.org. ..