Javascript嵌套函数调用始终返回true值

Question

The following javascript code works:

const numbers = [10, 15, 20, 25, 30];

function nesting(array, iteratorFunction) {
  let f2 = function(b){
        return false;
      }
  return array.filter(
    function(a){
      return f2(a)
    }   
   );
}

The result is [] when f2 returns false , and [10, 15, 20, 25, 30] if f2 is changed to return true .

Why doesn't it work correctly after this direction substitution where I simply replaced the variable f2 with the original definition of f2?

function nesting(array, iteratorFunction) {
  return array.filter(
    function(a){
      return function(b){
        return false;
      }
    }   
   );
}

It returns [10, 15, 20, 25, 30] whether the inner function returns true or false . Why? What piece of knowledge of javascript am I missing?

Answer 1

The codes are not equivalent.

In the first example, you're calling f2 , then using what it returns to filter.

In the second example, you never call the function ; you're returning the uncalled function in the filtering function. Since functions themselves are truthy, it allows all elements, regardless of what the function returns.

To make it equivalent, you need to call the inner function:

. . . 
function(a){
  return (function(b){
    return false;
  })(a); // Add parenthesis to call
}
. . . 

Answer 2

In the second snippet you never execute the f2 function

function nesting(array, iteratorFunction) {
  return array.filter(
    function(a){
      return function(b){
        return false;
      }(a)
    }   
   );
}