getting wrong values looping through array for a particular value in javascript

Question

I have a function that is supposed to loop through an array and count the values in the array that are true. in the example below, I am expecting a value of 4, since the values 6, 3, 30, and 7 are all truthy values while 0 is false.

function countTruthy(arr) {
    var arrayLength = arr.length;
    var truthy = 0;

    for (var i = 0; i < arrayLength; i++) {
    if(arr[i] == true) {
        truthy++
    }
  }
  
  return console.log(truthy)
}

countTruthy([6, 3, 0, 30, 7])

But the code above doesn't work and I keep getting 0 instead

Michael M. · Accepted Answer · 2022-10-29 15:06:19Z

4

While all non-zero numbers are truthy, it is not equal to true. Only 1 is equal to true. To double-check this, just try running 60 == true in your console and it will return false, however, if you run if (60) console.log('hi') it will print 'hi'.

tl;dr: Truthy values are not necessarily equal to true.

To fix this, just check if the value is 0 instead. Like this:

function countTruthy(arr) {
  return arr.filter(x => x != 0).length;
}

console.log(countTruthy([6, 3, 0, 30, 7]))

Also, your function can be drastically reduced with .filter(). Just filter out all the zero values then return the length of the resulting array.

edited Oct 29, 2022 at 15:06

answered Oct 29, 2022 at 14:43

Michael M.

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trincot · Accepted Answer · 2022-10-29 14:54:19Z

3

Whether a value is "truthy" (like 6 is truthy, but not really equal to true) has a meaning that goes beyond just numbers, and besides 0, also NaN is a falsy value in the numeric data type domain. To accurately count truthy values, you can use Boolean as call back, like this:

const countTruthy = arr => arr.filter(Boolean).length;

console.log(countTruthy([6, 3, 0, 30, 7]));  // 4
console.log(countTruthy([6, 3, NaN, 30, 7]));  // 4
console.log(countTruthy(["a", "b", "", "c", "d"]));  // 4
console.log(countTruthy([true, true, false, true, true]));  // 4
console.log(countTruthy([{}, undefined, null]));  // 1

edited Oct 29, 2022 at 14:54

answered Oct 29, 2022 at 14:48

trincot

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user11877521 · Accepted Answer · 2022-10-29 15:01:06Z

2

As @Michael mentioned above, 0 is false but not all truthy values are equal to true. With that said, here is how you can achieve the desired result.

function countTruthy(arr) {
  var arrayLength = arr.length;
  var truthy = 0;

  for (var i = 0; i < arrayLength; i++) {
    if (arr[i]) {
      truthy += 1;
    }
  }

  return console.log(truthy);
}

countTruthy([6, 3, 0, 30, 7])

answered Oct 29, 2022 at 15:01

user11877521

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