对 { } 中的 function 声明感到困惑

Question

 var a; if (true) { a = 5; function a() {} a = 0; console.log(a) } console.log(a)

I saw the code above, a function is declared in {}. I think it would print 0 0, but it prints 0 5

Answer 1

The following happens:

(1) There exist two variable declarations a , one inside the block and one outside of it.

(2) The function declaration gets hoisted, and bound to the inner blocks variable.

(3) a = 5 is reached, which overrides the block variable.

(4) the function declaration is reached, and the block variable is copied to the outer variable. Both are 5 now.

(5) a = 0 is reached, which overrides the block variable. The outer variable is not affected by this.

 var a¹;
 if (true) {
   function a²() {} // hoisted
   a² = 5;
   a¹ = a²; // at the location of the declaration, the variable leaves the block      
   a² = 0;
  console.log(a²)
}
console.log(a¹);

This is actually not really part of the specification, it is part of the web legacy compatibility semantics , so don't declare functions inside blocks and don't rely on this code to behave in this way .

This is also explained here