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Beginner JavaScript for loop

I was wondering if anyone knows the reason for [i-1] in the fourth line when you also have i++ in the second line? Thank you for any help! (It's from the book "JavaScript" by Vodnik and Gosselin.)
if (figureCount === 3) {
for (var i = 1; i < 4; i++) {
filename = "images/IMG_0" + photoOrder[i] + "sm.jpg";
currentFig = document.getElementsByTagName("img")[i - 1];
currentFig.src = filename;
}//end of for loop

It because document.getElementsByTagName returns an HTMLCollection(similar to array) hence. So accessing the 1st(and subsequent) img tag on the page are done through setting i-1

document.getElementsByTagName return a HTMLCollection which is not an array but an array like object. So to access any element from that collection you can pass the index.
document.getElementsByTagName("img")[i - 1] is creating a collection of all the img tags & it is accessing specific element in that collection by passing the index [i-1]
In the below example [1] is trying to access the second element from the collection
var getAllDiv = document.getElementsByTagName('div');
console.log(getAllDiv[1].innerHTML)
<div>1</div>
<div>2</div>

Some developers get confused with the for loop operators logic instead of doing it correctly:
for (var i = 0; i < 3; i++) {
they decided to add some extra processing to the mix ( which isn't that big of a deal ) but iv'e fired developers for less.
CurrentFig is using i - 1 because it appears there is a prepended img element so the developer also chose to select it as well instead of selecting the exact elements that he needs.

looping through html and deleting not found

So, i'm looping through the document body searching for all "#result>tbody>tr" objects. I'm then trying to search within those elements to see if any of the names exist, if they do, I would like to delete them. I got it working if I don't loop through the names and use a single object. If I try and loop through the names, it only loops four times and then nothing else happens.
Any ideas?
Edit: Currently there are 30 objects the first loop loops through. When I add the second loop into the mix to see if the sub-objects exist, it will only loop through four and than break the loop. Hope that explains better.
Example: https://jsfiddle.net/8c9p7bp5/1/
var dlList = document.querySelectorAll("#result>tbody>tr");
for (var i = 0, len = dlList.length; i < len; i++) {
var names = ['Test', 'Router', 'IP', 'Mod'];
for (var j = 0, len = names.length; j < len; j++) {
var vi = dlList[i].querySelector("td>br>img[title^='" + names[i] + "']");
if(!dlList[i].contains(vi)) {
//dlList[i].remove();
console.log(dlList[i]);
}
}
}

First mistake of your code that you are using same limit for you nested loop "len", in the first iteration the value becomes 4 that's why it will break the parent loop.

You initialize j but never use it in the inner loop.
Try this instead:
var vi = dlList[i].querySelector("td>br>img[title^='" + names[j] + "']");
Also, you should use a different variable than len for the inner loop to avoid running into variable scope issues.

Javascript: attempting to fill an array with a prompt but prompt is not displaying

I am attempting to call a method that asks the user how many entries they would like to make into an array and then prompts the user for each entry. I know this is likely a simple error but I cannot tell why my prompt is not working.
<script>
function testScore(){
var numberofScores = prompt("enter the number of scores:","");
var scores = new Array();
var whichScore=1;
for(var i=0; i<numberofScores; i++; whichScore++){
score[i]=prompt("enter score "+whichScore+":");
}
}
</script>
<a href="" onclick="testScore()">
Start Test score script
</a><br>

A loop is configured with 3 sections and thus two semi-colons. You had whichScore++ in a 4th section after adding a third semi-colon. You could have added it to the end of the configuration with a comma. But, adding it to the loop body, and not part of the loop declaration is cleaner. That said, the variable is not even needed. Just use (i + 1) and note that we're not modifying i here, we're just using an offset of it for display purposes only.
Also, in the loop: score[i], needs to be scores[i] and your <a> element should have an href="#" instead of an empty attribute.
Lastly, don't use inline HTML event handling attributes as they:
Make the code more difficult to read and lead to code duplication.
Cause global wrapper functions to be created around your supplied
attribute value that alter the binding of this in your function.
Don't follow the W3C DOM Event standard.
Use .addEventListener() in JavaScript instead:
// When the DOM content is ready
window.addEventListener("DOMContentLoaded", function(){
// Get a reference to the hyperlink and create a click event handler for it:
document.getElementById("makeScores").addEventListener("click", testScore);
function testScore(){
var numberofScores = prompt("enter the number of scores:","");
var scores = new Array();
for(var i = 0; i < numberofScores; i++){
scores[i] = prompt("enter score " + (i + 1) + ":");
}
console.log(scores);
}
});
Start Test score script

Here is a JSFiddle using your code. You have a lot going on here
https://jsfiddle.net/tcoedqkf/
First off, your for loop needs to have a comma besides the increments (although for readability I would do it in the for loop)
for(var i=0; i<numberofScores; i++,whichScore++){
Your variable name in the for loop is incorrect (missing an S)
scores[i]=prompt("enter score "+whichScore+":");

<script>
function testScore(){
var numberofScores = prompt("enter the number of scores:","");
var scores = new Array();
var whichScore=1;
for(var i=0; i<numberofScores; i++, whichScore++){
scores.push(prompt("enter score "+whichScore+":"));
//or
//scores[i]= (prompt("enter score "+whichScore+":"));
}
}
</script>
<a href="" onclick="testScore()">
Start Test score script
</a><br>
your variable is scores
Use Array.push, since you did not declare the array length in array constructor. Edit : You can also use scores[i]. It will work as well.

for only have three sections (separated by a semicolon ;): initialization, the condition and the incrementation. If you want to initialize or increment more variables use a comma ,. Like this:
for(var i = 0; i < numberofScores; i++, whichScore++) {
// ...
Since whichScore is basically just i + 1 you won't need to have two variables for that, just i will do:
for(var i = 0; i < numberofScores; i++) {
score[i] = prompt("enter score " + (i + 1) + ":");
// ...
Note that the parenthesis in (i + 1) are necessary so the numbers are added instead of concatenated.

Get array values and put them into different divs with a for loop?

I'm trying hard to learn javascrip+jquery on my own, and also trying to learn it right. Thus trying to enforce the DRY rule.
I find myself stuck with a problem where I have an array,
var animals = [4];
a function,
var legs = function(amount){
this.amount = amount;
this.body = Math.floor(Math.random()*amount)+1;
}
and an evil for loop. I also have 5 div's called printAnimal1, printAnimal2 and so on.. In which I wish to print out each value in the array into.
for(i = 0; i < animals.length; i++){
animals[i] = new legs(6);
$(".printAnimal"+i).append("animals[i]");
}
I feel as if I'm close to the right thing, but I cant seem to figure it out. I also tried something like this:
for(i = 0; i < animals.length; i++){
animals[i] = new legs(6);
$this = $(".printAnimal");
$(this+i).append("animals[i]");
}
But one of the problems seem to be the "+i" and I cant make heads or tails out of it.
I also know that I can simply do:
$(".printAnimal1").append("animals[i]");
$(".printAnimal2").append("animals[i]");
$(".printAnimal3").append("animals[i]");
...
But that would break the DRY rule. Is it all wrong trying to do this with a for loop, or can it be done? Or is there simply a better way to do it! Could anyone clarify?

Your first attempt should be fine, as long as you take "animals[i]" out of quotes in your append() call ($(".printAnimal"+i).append(animals[i]))
Also, I assume you declared var i; outside your for loop? If not, you'll want to declare it in your for loop (for(var i=0....)
EDIT: problems with your fiddle
you never call startGame()
you didn't include jQuery
you can't (as far as I know) append anything that isn't html-- in your case, you're trying to append a js object. What do you want the end result to look like?
http://jsfiddle.net/SjHgh/1/ is a working fiddle showing that append() works as you think it should.
edit: forgot to update the fiddle. Correct link now.
EDIT: reread your response to the other answer about what you want. http://jsfiddle.net/SjHgh/3/ is a working fiddle with what you want. More notes:
You didn't declare new when you called DICE
you have to reference the field you want, (hence dices[i].roll), not just the object

Just a few comments:
This is declaring an array with only one item and that item is the number 4
var animals = [4];
In case you still need that array, you should be doing something like:
var animals = []; // A shiny new and empty array
and then add items to it inside a for loop like this:
animals.push(new legs(6)); //This will add a new legs object to the end of the array
Also, what is the content that you are expecting to appear after adding it to the div?
If you want the number of legs, you should append that to the element (and not the legs object directly).
for(i = 0; i < animals.length; i++){
animals.push(new legs(6));
$(".printAnimal"+i).append(animals[i].body);
}

Adding another answer as per your comment
var i, dicesThrown = [];
function throwDice(){
return Math.ceil(Math.random() * 6);
}
//Throw 5 dices
for (i=0 ; i<5 ; i++){
dicesThrown.push( throwDice() );
}
//Show the results
for (i=0 ; i<5 ; i++){
$("body").append("<div>Dice " + (i+1) + ": " + dicesThrown[i] +"</div>");
}

JavaScript array.sort() Issue

drafting up a quick listing tool to list local kids baseball teams. Takes a couple of inputs and writes to a text field. There's some validation and whatnot too, but that's out of scope and doesn't seem to be impacting things.
Problem is, I'm having trouble figuring out how to "capture" the existing text, add the new inputs and sort the whole lot, before writing the new result to the paragraph element (effectively replacing it).
So far I have:
var LeagueTeams = [];
var IndividualTeam = '';
LeagueTeams.push(document.forms[0].TeamName.value);
LeagueTeams.push(document.getElementById('TeamList')
LeagueTeams = LeagueTeams.sort();
for (j = 0; j < LeagueTeams.length; j++) {
IndividualTeam = LeagueTeams.pop();
IndividualTeam = IndividualTeam + '' + \n;
document.forms[0].TeamName.value += IndividualTeam;
}
What I end up getting is my input, and then an array of my input PLUS the previous contents, with a couple of line breaks. Setting the operator to = instead of =+ stops it from printing to the array at all.
i.e.
Enter: a
Text area: a
Then enter: b
Text area: a ab
(etc)

OK, now that we have a better idea of what you're trying to do, here's some code that will do that:
HTML:
<label>Enter Team Name: <input id="newTeam" type="text"></label>
<button id="add">Add</button><br>
All Teams:<br>
<textarea id="allTeams" rows="40" cols="40"></textarea>
Javascript (plain javascript, no framework, called after page is loaded):
var teamList = ["Dodgers", "Mets", "Giants"];
document.getElementById("add").onclick = function() {
var input = document.getElementById("newTeam");
if (input.value) {
teamList.push(input.value);
}
updateTeamList();
input.value = "";
}
function updateTeamList() {
teamList.sort();
var o = document.getElementById("allTeams");
o.value = teamList.join("\n");
}
updateTeamList();
And, you can see it working here: http://jsfiddle.net/jfriend00/HkhsL/
Comments on your existing code:
I'm not sure I understand what you're trying to do overall, but do you realize that this loop is going to have problems:
for (j = 0; j < LeagueTeams.length; j++) {
IndividualTeam = LeagueTeams.pop();
IndividualTeam = IndividualTeam + '' + \n;
document.forms[0].TeamName.value += IndividualTeam;
}
Each time you do LeagueTeams.pop() you are reducing the length of the array and you're continually comparing to LeagueTeams.length in the for loop. This will only get half way through the array because each time through the loop, you increment j and decrement LeagueTeams.length which means you'll only get half way through the array.
If you intend to iterate all the way through the array in your for loop, you should use this version that gets the length once initially and simplifies the code in the loop:
for (j = 0, len = LeagueTeams.length; j < len; j++) {
document.forms[0].TeamName.value += LeagueTeams.pop() + '\n';
}
or perhaps even better, this version that doesn't even use j:
while (LeagueTeams.length > 0) {
document.forms[0].TeamName.value += LeagueTeams.pop() + '\n';
}
Then further, I see that you're trying to use LeagueTeams.sort() on an array that has both strings in it and DOM object references. What are you trying to do with that sort because the built-in sort function does a lexigraphical sort (e.g. alpha) which will do something odd with a DOM reference (probably sort by whatever toString() returns which may be object type)?
If you want to sort the input by team name, then you would need to put both team name and the DOM reference into an object, insert that object into the array as one unit and then use a custom sort function that would sort by the name in the object. As your code is written above, you see to be using document.getElementById('TeamList') which is the same for all teams so I'm not sure why you're putting it into the array at all.
If you can show your HTML and a more complete version of your code, we could help further. What you have above is just a non-working piece of code and we don't know what your HTML looks like that it's trying to operate on.
FYI, there are several syntax errors in the code you posted, so this can't be running code:
Missing paren at the end of this: LeagueTeams.push(document.getElementById('TeamList'))
Missing quotes around \n: IndividualTeam = IndividualTeam + '' + '\n';

If you are just trying to make a list of the teams, try something like:
<script type="text/javascript">
function addTeam(form) {
var para = document.getElementById('teamList');
var teams = para.innerHTML.split(/<br\s*[\\]?>/);
teams.push(form.teamName.value);
para.innerHTML = teams.sort().join('<br>');
}
</script>
<form action="">
<input type="text" name="teamName">
<input type="button" value="Add team" onclick="addTeam(this.form)">
</form>
<p id="teamList"></p>
You may be using different elements or layout, but the strategy should be about the same. If you are making a set of options for a select, things are a little easier.