Array loop in javascript

Use it like this,

Array indexes should start from 0, that is why I have introduced another variable j=0

var numbers = [];
var sum = 0;
for (var i = 0, j = 0; i <= 100; i = i + 3, ++j) {
    numbers[j] = i;
}

Update

First Issue: In your code, ie. below code of yours,

for (var i = 0; i <= 100; i = i + 3) {
    numbers[i - 1] = i;
}

In the first iteration,

i = 0;
numbers[0-1] = i // i.e numbers[-1] = 0; 

and in your second loop, you are starting the index from 0

for (var index = 0; index < 100; index++) {

Second issue:

Also, if you don't use a sequential counter to fill the Array, you will end with undefined values for the ones you did not fill.

If you notice, the output after the loop, it says numbers.length = 99 which is wrong it will not have that many items in it.

Third Issue:

In below code, even if you introduce a sequential counter, this is still wrong

for (var i = 0; i <= 100; i = i + 3) {
    numbers[i - 1] = i;
}

because i should start with 3 instead of 0, otherwise you will end up with 34 elements in the array because numbers[0] will be 0;

Fourth Issue:

In this code,

for (var index = 0; index < 100; index++) {
    document.write("<br/>" + numbers[index]);
    sum = sum + i;
}

You don't actually have to loop it till 100, you already have the numbers array filled, so you just need to use numbers.length, like this

var len = numbers.length;
for (var index = 0; index < len; index++) {
    document.write("<br/>" + numbers[index]);
    sum = sum + i;
}

A better way to write this

var numbers = [];
for (var i = 3, j=0; i <= 100; i = i + 3, j++) {
    numbers[j] = i;
}

var sum = numbers.reduce((a, b) => a+b);
console.log(sum);

The line var sum = numbers.reduce((a, b) => a+b); uses Array.reduce() method.

There are few issues in your code.

for (var i = 0; i <= 100; i = i + 3) {
    numbers[i - 1] = i;
}

1: array is 0 based. so first insertion into the array goes for a toss.

2: the number array created will have skipping index like 3, 6 ,9

for (var index = 0; index < 100; index++) {
    document.write("<br/>" + numbers[index]);
    sum = sum + i;
}

3: Here you are iterating index till 100 , you should iterate it till the length of the numbers array only.

when index is 1,2 number[index] will become undefined.

4: sum = sum + i (i ??????)

You should try like this or you can also use push()

var numbers = [];
var sum = 0;
for (var i = 0,j=0; i <= 100; i = i + 3, j= j+1) {
    numbers[j] = i; // array is 0 based.
  
}



for (var index = 0; index < numbers.length; index++) {
    document.write("<br/>" + numbers[index]);
    sum = sum + numbers[index];
}

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

Indexes in an array begin with zero.

for (var i = 0; i <= 100; i = i + 3) {
    numbers[i - 1] = i; // In the first iteration, there will be numbers[-1] = i;
}

You have several issues i suppose.

var numbers = [];
var sum = 0;
for (var i = 0; i <= 100; i = i + 3) {
    numbers.push(i);
}

for (var index = 0; index < numbers.length; index++) {
    document.write("<br/>" + numbers[index]);
    sum = sum + i;
}

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

Also for array you can use:

for (var i in array) {
console.log(array[i]);
}

And I'm pretty sure, that array of number sequence is absolutely useless, if there is no other information in it.

Try this

var numbers = [];
var sum = 0;
for (var i = 0; i <= 100; i = i + 3) {
    numbers[(i-3)/3] = i;
}

for (var index = 0; index <  numbers.length; index++) {
    document.write("<br/>" + numbers[index]);
    sum = sum + numbers[index];
}

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

Here is the fiddle i tried https://jsfiddle.net/4ncgnd7c/

This should work using a single loop

var numbers = [];
var sum = 0;
for (var i = 3; i <= 100; i = i + 3) {
    numbers[i] = i;
    document.write("<br/>" + i);
    sum = sum + i;
}

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