Why does this Javascript function always return true?

Question

I built a function every that should iterate over an array and return true if the action (for ex. element < 10) performed on all the elements is true. Here's my code:

function every(array, action) {
  var trueOrFalse = true
  for (var i = 0; i < array.length; i++)  
    trueOrFalse = trueOrFalse && action(array[i]);
  if (trueOrFalse = true) return true;
  else  return;
}
array1 = [1,2,3,4,5,6,7,8,9,10,11]
console.log(every(array1, function(element) {
  return element < 10 
}))

I don't see anything wrong. With array1 it returns true even if it contains numbers > 10. Where's the problem?

Thanks

Optimization idea: you don't need to iterate the rest of the elements once you found the first one that does not satisfy your condition. It's called "short circuiting". As a bonus, this will also eliminate your boolean calculation code and the bug which hides there. — Sergio Tulentsev
– Sergio Tulentsev, Commented May 10, 2017 at 15:20
There is a built-in array method for this: Array.prototype.every. — glhrmv
– glhrmv, Commented May 10, 2017 at 15:22
Is anyone going to mention, why not use the built in array.prototype.every() method? — mhodges
– mhodges, Commented May 10, 2017 at 15:23
Don't use the variable at all. The first time that action gives a falsey result, return false immediately. Then put return true after the loop. for (var i = 0; i < array.length; i++) { if (!action(array[i])) return false } return true — user1106925
– user1106925, Commented May 10, 2017 at 15:25
Yes, I considered that option. Was just curious to know why mine dind't work. — Federico
– Federico, Commented May 10, 2017 at 15:34

mika · Accepted Answer · 2017-05-10 15:18:14Z

4

if (trueOrFalse = true) return true;

should be

if (trueOrFalse == true) return true;

answered May 10, 2017 at 15:18

mika

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4 Comments

Mark Reed Over a year ago

This. trueOrFalse = true is an assignment. It sets trueOrFalse to true - and then incidentally returns the value it just set, so the condition is always true.

Jeremy Thille Over a year ago

if (trueOrFalse == true) should be if (trueOrFalse)

Prusse Over a year ago

Why not just return trueOfFalse?

Federico Over a year ago

@Mark Reed Thanks! For a beginner like me that was the most helpful comment.

Mr Mystery Guest · Accepted Answer · 2017-05-10 15:28:53Z

1

You need to evaluate trueOrFalse is equal to true For which you need the double equals

if (trueOrFalse == true) return true;

Other wise you'll just be making the value of trueOrFalse the same as true

Bonus points:

 if (trueOrFalse === true) return true;

Using three equal signs is evaluating exactly the same type and value. That isn't required here but is useful to know.

edited May 10, 2017 at 15:28

answered May 10, 2017 at 15:18

Mr Mystery Guest

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4 Comments

Mark Reed Over a year ago

could use more explanation than just a line of corrected code.

Mr Mystery Guest Over a year ago

I assumed it was a typo and didn't require it

mhodges Over a year ago

@MrMysteryGuest If you believed it was a typographical error, you should flag the answer as such and move on.

Mr Mystery Guest Over a year ago

@mhodges I'll do that from now on.

Master Yoda · Accepted Answer · 2017-05-10 15:27:13Z

0

Your condition is using an incorrect operator, you should be using the == operator.

You can use if (trueOrFalse == true) return true;

OR

you can write it as if (trueOrFalse) return true; which will still evaluate it as if( true )

answered May 10, 2017 at 15:27

Master Yoda

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Comments

Cameron Wilby · Accepted Answer · 2019-11-30 10:25:44Z

0

You can remove the if statement and rely on good old boolean algebra!

function every(array, action) {
  var trueOrFalse = true
  for (var i = 0; i < array.length; i++)  
    trueOrFalse = trueOrFalse && action(array[i]);
  return trueOrFalse;
}
array1 = [1,2,3,4,5,6,7,8,9,10,11]
console.log(every(array1, el => el < 10));

answered Nov 30, 2019 at 10:25

Cameron Wilby

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