Why don't use filter like:

const arr = [false,7, "ate", "", false, 9];
console.log(arr.filter(el => el !== false));

Reference:

• Array.prototype.filter()

You can't iterate upwards AND shorten your array like this; splice has the side effect of modifying the original array.

Consider what happens in the case [false, false, true]

1. i = 0, length = 3, arr = [(false), false, true] -> arr becomes [false, true]
2. i = 1, length = 2, arr = [false, (true)]-> arr stays [false, true]

(parenthesis denotes current target)

Solutions:

• Iterate down, not up

  for (let i = arr.length - 1; i >= 0; --i) if (!arr[i]) arr.splice(i, 1);
  

• Use Array.prototype.filter instead (this will also improve readability)

  arr = arr.filter((item) => !!item);
  

The .filter() approach as suggested by Rossaini or PaulS is definitely a better path to follow. However, your approach works if you work through the array backwards:

const test=[7, "ate", "", false, 9];

function bouncer(arr) {
  if (!arr.length) return [];
  for (let i=arr.length-1; i--;){
    if(!arr[i]){
        arr.splice(i, 1)
      }
  }
  return arr;
}

console.log(bouncer(test));
console.log(bouncer([]));
// solution with filter:
console.log(test.filter(e=>e));

You need to consider that .splice() actually changes the source array you are working on. Using an increasing index i you will skip an entry each time an element is deleted from the array. By walking through the array backwards (from the end to the beginning) as done with for (let i=arr.length; i--;) you will avoid this problem.