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Can someone please tell me why the length of secondArrayis 5 instead of 10?

The result I am looking for is for all elements to be popped off so the secondArray has an empty. However, it seems that only half of them are being popped off, even though I have set the (condition) of the for loop to go through the entire array. Can someone point out why this is?

Please Note: I understand this is not the only/or ideal way to remove elements from an array. This is simply some practice I am doing with for loops and Array methods.

my result looks like this after executing the code:

secondArray = 1,2,3,4,5,6,7,8,9,10

secondArray = 1,2,3,4,5

var secondArray = [1,2,3,4,5,6,7,8,9,10];

document.write("secondArray = " + secondArray)

for(i = 0; i < secondArray.length;  i++){
    secondArray.pop(); 
}

document.write("<br/>"+ "secondArray = " + secondArray)

Can someone please tell me why the length of secondArrayis 5 instead of 10?

The result I am looking for is for all elements to be popped off so the secondArray has an empty. However, it seems that only half of them are being popped off, even though I have set the (condition) of the for loop to go through the entire array. Can someone point out why this is?

Please Note: I understand this is not the only/or ideal way to remove elements from an array. This is simply some practice I am doing with for loops and Array methods.

my result looks like this after executing the code:

secondArray = 1,2,3,4,5,6,7,8,9,10

secondArray = 1,2,3,4,5

var secondArray = [1,2,3,4,5,6,7,8,9,10];

document.write("secondArray = " + secondArray)

for(i = 0; i < secondArray.length;  i++){
    secondArray.pop(); 
}

document.write("<br/>"+ "secondArray = " + secondArray)

• javascript
• arrays
• for-loop
• methods

Share Improve this question Follow edited Aug 26, 2019 at 11:07 Vadim Kotov 8,28488 gold badges5050 silver badges6363 bronze badges asked Jul 17, 2016 at 19:53 zzgooloozzgooloo 7511 gold badge33 silver badges99 bronze badges 3

• 1 What are you expecting to happen? Because it's doing exactly what you're telling it to do. – Niet the Dark Absol Commented Jul 17, 2016 at 19:56
• 1 Every time you loop secondArray.length changes because you keep popping off elements – j08691 Commented Jul 17, 2016 at 19:57
• If you want to empty out secondArray, then just do secondArray.length = 0;. – user663031 Commented Jul 17, 2016 at 20:10

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5 Answers 5

Sorted by: Reset to default 4

You should rewrite this as:

while (secondArray.length) secondArray.pop();

I will not address the question of why you are trying to empty an array by popping off all its elements, instead of just setting the length to 0.

secondArray.length gets calculated for each iteration. On each iteration secondArray has one item less.

To pop() all the elements save secondArray.length and use the variable:

var firstArray = [];
var secondArray = [1,2,3,4,5,6,7,8,9,10];

document.write("secondArray = " + secondArray)

var len = secondArray.length;

for(i = 0; i < len;  i++){
    secondArray.pop(); 
}

document.write("<br/>"+ "secondArray = " + secondArray)

Note

There are many other and better ways to empty an array as OP aware of already. The answer addresses the mon mistake of thinking that a for loop condition is calculated only at the first iteration.

var firstArray = [];

var secondArray = [1,2,3,4,5,6,7,8,9,10];

console.log(secondArray)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] //nothing change

 //first loop runs
 for(i = 0; i < secondArray.length;  i++){ 
    // removing one element and now only 9 elements left 
    // means length have changed 
    // it's is doing his job as expacted
   secondArray.pop();         
   console.log(secondArray)
 }
// first run when length was 9  [1, 2, 3, 4, 5, 6, 7, 8, 9]
// second run when length was 8  [1, 2, 3, 4, 5, 6, 7, 8]
// third run when length was 7  [1, 2, 3, 4, 5, 6, 7]
// fourth run when length was 6  [1, 2, 3, 4, 5, 6]
// fifth run when length was 5 [1, 2, 3, 4, 5]
// and here loops end because of given condition 

Because you are starting the loop at 0 and as long as the length of the array is less than your length the loop will continue. How every once it reaches 6 the i variable bee grater than the length of the array because you are popping a value out of the array with each run through the loop.

array = 10, 9, 8, 7, 6, 5, 4

var = 0, 1, 2, 3, 4, 5, 6

It terminates when your var[6] is greater than your array[4].

The pop method removes the last element from an array and returns that value to the caller.

It means the result is the expected.

If you want to clear the array put the initial array length in a var to make it static, but I can see two ways to do that better.

1. Just assign array.length to 0
2. Assign the array to be an empty array.

Check the docs

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