Falsy Bouncer and filter method

JavaScript
#1

Tell us what’s happening:
How does this code work?
How the filter method knows that the element pass is truthy and not falsy?

function bouncer(arr) {
return arr.filter(Boolean);
}
Your code so far


function bouncer(arr) {
  // Don't show a false ID to this bouncer.

//Below is my code
/*
let arr1 = [];

arr.filter(function(x) {
  if(!!x === true) {
    arr1.push(x);
  }
});

return arr1;
*/
}

bouncer([7, "ate", "", false, 9]);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.110 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/basic-algorithm-scripting/falsy-bouncer

#2

The function are passing as an argument to your .filter

function(x) {
  if(!!x === true) {
    arr1.push(x);
  }
}

Is only replaced by another native function called Boolean…
The Boolean returns true if if the argument you pass to it is truthy.and false otherwise.

PS: You don’t need to manually push into arr1..filter implicitly returns a copy of filtered array…

function bouncer(arr) {
  return arr.filter(function (x) {
    return !!x;
  });
}
1 Like
#3

Actually, the !! is not needed, because filter will coerce the values to Booleans when returned. Simply returning x will have the same effect.

2 Likes
#4

Is that true? I would think that was an implementation detail of the engine?

Array.prototype.filter ( callbackfn [ , thisArg ] )

Let selected be ToBoolean(Call(callbackfn, T, «kValue, k, O»)).

SpiderMonkey JS::ToBoolean

#5

No, it’s in the spec, conditionals convert to a Boolean to allow them to evaluate properly, the semantics don’t work otherwise — in a strongly typed language, this wouldn’t be an issue, because putting non-boolean expressions in there is impossible, but JS isn’t strongly typed so it just tries to make sure the program won’t explode via coercion

#6

I think I need more of this to understand filter and boolean.

thanks man

#7

Thanks man :slight_smile:

#8

I posted a video on this same topic yesterday… Hope this helps… :slight_smile:

1 Like
#9

Thanks for the video adityaparab