I am trying to print a list of alternate days but getting a lot undefined

I am trying to print a list of alternate days in Javascript. I am getting the required output but there are a lot undefined getting printed. Why?

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function alternate_days(name) {
  let days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']

  if (name % 2 == 0) {
    for (let i = 0; i <= 7; i++) {
      console.log(days[name])
      name = name + 2
    }
  } else {
    for (let i = 0; i <= 7; i++) {
      console.log(days[name])
      name = name + 2
    }
  }
}

console.log(alternate_days(2));

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Expected Output:

Tuesday
Thursday
Saturday

In your example, you enter the function with name being 2, and the following two line get executed 8 times, the for loop is set from 0 to 7 inclusively.

console.log(days[name])
name = name + 2 

So, writing it by unwrapping the loop, you'd get this:

console.log(days[name])
name = name + 2 
console.log(days[name])
name = name + 2 
console.log(days[name])
name = name + 2 
console.log(days[name])
name = name + 2 
console.log(days[name])
name = name + 2 
console.log(days[name])
name = name + 2 
console.log(days[name])
name = name + 2 
console.log(days[name])
name = name + 2 

And changing the name variable to its value when used:

console.log(days[2])
name = name + 2 
console.log(days[4])
name = name + 2 
console.log(days[6])
name = name + 2 
console.log(days[8])
name = name + 2 
console.log(days[10])
name = name + 2 
console.log(days[12])
name = name + 2 
console.log(days[14])
name = name + 2 
console.log(days[16])
name = name + 2

So, after the third iteration the name variable is above 6. 6 being the biggest index in your days array, the others are therefor undefined.

There are 6 undefined logged because of the 5 seen here, and the last one being the console.log(alternate_days(2)); since alternate_days does not return anything.

Se other answer for why.

You need to move the test if (name % 2 == 0) { and not change the name

or just filter

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let days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']

const alternate_days = which => days.filter((day,i) => which === 0 && i === 0 || i > 0 && i%which ===0)

console.log(alternate_days(1));
console.log(alternate_days(2));
console.log(alternate_days(3));

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The for loop always loops 8 times, regardless of the days you want to alternate. That causes the index you access to go over the array boundary with the days.
If you want to simply get alternating days of the week, without repetitions, you would have to limit the stop condition of the for loop (or use a while). For example:

while(name < days.length) {
   console.log(days[name]);
   name = name + 2;
}

This will return, with your input case: Tuesday, Thursday, Saturday.

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function alternate_days(name) {
  let days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
  const result = days.filter((day,i) => i >= name && i % 2 == 0);
  result.forEach(day => console.log(day));
}

alternate_days(2);

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