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JavaScript and WordPress: button click not found by addEventListener

To prevent answers like: 'is the JavaScript file loaded?' -> Yes, it is loaded, at the footer part of the page! I have checked that with a simple message to the console, which is displayed!
But:
I've got a page with a button:
<button id="portfolio-posts-btn">Load portfolio related blog posts</button>
And a file main.js:
var portfolioPostsBtn = document.getElementById('portfolio-posts-btn');
var portfolioPostsContainer = document.getElementById("portfolio-posts-container");
if (portfolioPostsBtn) {
portfolioPostsBtn.addEventListener("click", function () {
console.log("the button was clicked!");
});
}
The text the button was clicked! should be displayed in the console, but it stays empty!
Apparently, the button click is not recognized, and thus, the var portfolioPostsBtn is false, or NULL... -> the method addEventListener() is not fired ?
I don't see the cause for this; I checked the spelling, should I use single or double quotes? Please help?
Thank you!

I've had this happen to me before, since theres two ways to do this I just used the other.
The first is onclick="function()", this is used as an attribute inside the element. Ex:
function clicked(){
alert("button clicked");
}
<button onclick="clicked();">Press me</button>
exaplaination: When you add this attribute to this element and I do believe some others when the button is clicked the specified code inside the quotes of the attibute will run. It doesn't have to be a number, e.g. onclick="alert(12+4/2);". But this is more of HTML than JavaScript using this version
The other way is using what you've got which (to me) is a lot more difficult then it needs to be. Heres my example
var b = document.getElementById("btn");
b.addEventListener("click", blogged);
function blogged(){
alert("this post has been blogged");
}
<button id="btn">Blog it</button>
This side of things has more to do with JavaScript and Event listeners. But the problem with you're code is that you're putting the event listener after you call the if statement. Here's my solution
var portfolioPostsBtn = document.getElementById('portfolio-posts-btn');
portfolioPostsBtn.addEventListener("click", function(){
check();
});
function check(){
if(portfolioPostsBtn){
console.log("posted");
}
}
<button id="portfolio-posts-btn">press this to post<button>

Presumably you have made a decision not to use jQuery. You'll need to wrap your code in an event listener so that the code is executed when the DOM is ready.
document.addEventListener("DOMContentLoaded", function(event) {
var portfolioPostsBtn = document.getElementById("portfolio-posts-btn");
var portfolioPostsContainer = document.getElementById("portfolio-posts-container");
if (portfolioPostsBtn) {
portfolioPostsBtn.addEventListener("click", function () {
console.log("the button was clicked!");
});
}
});

The answer is found in the uploading of the file page-portfolio.php!
I found out that the id="portfolio-posts-btn", added later, was not updated - could be my mistake, or the SFTP upload extension in Brackets - I did not see an error message!
Anyway, the issue is solved!
One more question: "are there methods to check if an id exists?". That could make live easier!
All contributors, thank you for your answers!

Button.onclick automatically triggers, then will not trigger again

I have a script, which I'm using to try and display only one section of a webpage at a time.
function showMe(id){ clearPage(); changeDisplay(id, "block"); console.log(id)}
Currently, I'm using buttons to change which section is displayed.
var aBtn = document.getElementById("a-btn");
var otherBtn = document.getElementById("other-btn");
aBtn.onclick=showMe("a-btn-section-id");
otherBtn.onclick=showMe("other-btn-section-id");
However, when I load the page, the following happens:
I see the function attached to each button activate once in sequence in the console.
The page refuses to respond to further button inputs.
Testing with the console shows that showMe() and the functions it calls still all work properly. I'm sure I'm making a very basic, beginner mistake (which, hopefully, is why I can't find this problem when I Google/search StackOverflow/read event handling docs), but I'm at a loss for what that mistake is. Why would my script assume my buttons are clicked on load, and why won't it let me click them again?

You're calling the function an assign the value to onclick property instead of attach the function, try defining your onclick property as:
aBtn.onclick=function(){showMe("a-btn-section-id");};
otherBtn.onclick=function(){showMe("other-btn-section-id");};
Try the follow jsfiddle:
function showMe(id){ // some stuff..
console.log(id)
}
var aBtn = document.getElementById("a-btn");
var otherBtn = document.getElementById("other-btn");
aBtn.onclick=function(){showMe("a-btn-section-id");};
otherBtn.onclick=function(){showMe("other-btn-section-id");};
<input type="button" value="a-btn" id="a-btn"/>
<input type="button" value="other-btn" id="other-btn"/>
Hope this helps,

Using Document Ready on Click Event

I am still new to javascript.
I have an application that has two buttons on the page. One is a cpu_vs_player button that displays one game and the other is a player_vs_player button that displays a different game. The problem is that all the code is located in one application.js file. There is no need to load the player_vs_player on $(document).ready(function(){}); if I were to play cpu_vs_player.
Any ideas on how I can get them to load only if I chose that game? (I am only using one route with all the information being hidden / shown based on the click).

The document.ready is nothing more than the moment after the page has rendered and the document needs to be populated with event listeners. Frankly there are multiple way of skinning this cat.
You can either do the jQuery way where you keep javascript and HTML divided:
<button id="button1">cpu_vs_player</button>
<button id="button2">player_vs_player</button>
And for JavaScript:
Assuming you have a function for each gameplay:
function cpu_vs_player() {
// start the game
}
function player_vs_player() {
// need another player
}
Add event listeners the jQuery way:
$(document).ready(function() {
$("#button1").click(function() {
cpu_vs_player();
});
$("#button1").click(function() {
player_vs_player();
});
});
OR you could use the method #Techstone shows you, though you could do it more direct. It all works though.
<button onclick="javascript:cpu_vs_player();">cpu_vs_player</button>
<button onclick="javascript:player_vs_player();">player_vs_player</button>
Adding another option you can apply
In Javascript:
var Main = {
cpu_vs_player: function() {
alert("start cpu_vs_player");
},
player_vs_player: function() {
alert("start player_vs_player");
}
}
In your HTML:
<button onclick="javascript:Main.cpu_vs_player();">cpu_vs_player</button>
<button onclick="javascript:Main.player_vs_player();">player_vs_player</button>
And yes, there is more ... ;-)

image that your two button and js definition like below
function LetsRock(Playmate) {
....
}
<input type='button' value='cpu_vs_player' id='cpu_vs_player' onclick='javascript:LetsRock(this.id);' />
<input type='button' value='player_vs_player' id='player_vs_player' onclick='javascript:LetsRock(this.id);' />

Try to use the function with parameters (i.e. 0 to cpu v/s player, 1 to player v/s player), and send from the menu page to the $(document).ready(function(){});

Waiting for user input with jQuery and Javascript

I am trying to wait to take the user's input (clicking a button) before continuing with the program. A simplified version of the code is this:
$(document).ready(function(){
var answer = $(".question").on("click", "button", takeAnswer);
alert(answer);
});
//Code to set the next question once the user has answered the previous question
function takeAnswer(){
var answer = ($(".question button").data("answer"));
alert(answer);
return answer;
}
The HTML body simply contains this:
<p class="question"><button data-answer="1">Push me</button></p>
On loading, the HTML alerts [object Object]. once you click the button, the page alerts 1 as it is supposed to. I think the problem is that the return line is executing without awaiting the user's click. My research suggests that I need to use a callback function (there are several discussions on waiting for user input), but I'm confused because I thought that putting the takeAnswer code in a separate function would keep the subsequent code from executing.
For further context, my goal is to ask a series of choose-your-own-adventure style questions, changing later questions based on earlier user input. I intend to use jQuery to remove the previous question and place the new question, according to logic within the JavaScript file. Edit 1: I should also mention that I am pretty new to this and am trying to teach myself (if that's not already obvious).
Edit 3: Here is the full context in JSFiddle. http://jsfiddle.net/T7VhC/ The problem is the "undefined" error arising from line 144 of the JS.
Edit 2:
Eventually I want to add other buttons and capture which button was clicked. Here is a simplified example of another (dysfunctional because, I think, there is no callback) attempt to implement what I'm doing:
function takeAnswer(){
$(document).ready(function(){
$(".question").on("click", "button", function(){
return $(this).data("answer");
});
});
}
alert(takeAnswer());
HTML:
<body>
<p class="question"><button data-answer="1">Push me</button></p>
<p class="question"><button data-answer="2">Or push me</button></p>
</body>
Thanks.

For complete code, you could use:
$(document).ready(function(){
$(".question").on("click", "button", takeAnswer);
});
//Code to set the next question once the user has answered the previous question
$(takeAnswer);
function takeAnswer(){
var answer = $(".question button").data("answer");
alert(answer);
}

Try following
<script>
$(document).ready(function(){
var answer = $(".question").on("click", "button", takeAnswer);
takeAnswer();
});
//Code to set the next question once the user has answered the previous question
function takeAnswer(){
var answer = ($(".question button").data("answer"));
alert(answer);
return answer;
}
</script>

Making a basic Javascript calculator

jsfiddle demo
Bear with me, total newb here.
I'm trying to make a simple multiplication calculator, as a experimentation with Javascript.
The catch is that -
No libraries, just pure javascript.
Javascript must be unobtrusive.
Now, the problem arises, that it doesn't give the value out.
When I do this locally, answer has a value of NaN, and if you hit Submit it stays that way, BUT, if you press the back button, you see the actual result.
In the JSFiddle, much is not shown, except for the fact that it simply doesn't work.
Please tell me, is it even possible to make an unobtrusive calculator? How?
(PS. I was taking a bit of help from sciencebuddies, just to see basic syntax and stuff, but I found it can't be done without code being obtrusive)

I realize you're probably just getting started and don't know what to include, remove, and whatnot. But, good advice here, clearly label your elements so you can understand them, and pare it down to the smallest possible code you need for it to work (even less, so you can build it up).
Here is your code reworked:
HTML
<div>
<input type="text" id="multiplicand" value="4">
<input type="text" id="multiplier" value="10">
<button type="button" id="multiply">Multiply</button>
</div>
<p id="result">
The product is: <span id="product"> </span>
</p>
Javascript
window.onload = function(){
var button = el('multiply'),
multiplicand = el('multiplicand'),
multiplier = el('multiplier'),
product = el('product');
function el(id) {
return document.getElementById(id);
};
function multiply() {
var x = parseFloat(multiplicand.value) || 0,
y = parseFloat(multiplier.value) || 0;
product.innerHTML = x * y;
}
button.onclick = multiply;
};
http://jsfiddle.net/userdude/EptAN/6/
A slightly more sophisticated approach, with add/subtract/multiply/divide:
http://jsfiddle.net/userdude/EptAN/9/

You have to change the submit button so that it doesn't submit the form. Right now clicking "Submit" causes the form submits to the same page which involves a page reload.
Change the <input type="submit" id="submitt"> to <button type=button> and it should work.
You can probably do without the <form> element in the first place. That'll stop clicking enter in your text input from reloading the page.

Your example has a couple of problems:
The form still submits. After the JS changes the value, the submit will cause the page to reload, and that work you've done setting the answer value is wasted.
You're trying to do this stuff right away. In the header, none of the body has been parsed yet (and thus, the form elements don't even exist). You'll want to wait til the page is loaded.
The script hijacks window.onload. If you don't have any other scripts on the page, that's fine...but the whole point of unobtrusive JS (IMO) is that nothing breaks whether the script is there or not.
Fixed, we have something kinda like:
// Wrap this onload in an IIFE that we pass the old onload to, so we can
// let it run too (rather than just replacing it)
(function(old_onload) {
// attach this code to onload, so it'll run after everything exists
window.onload = function(event) {
// run the previous onload
if (old_onload) old_onload.call(window, event);
document.getElementById('Xintox').onsubmit = function() {
var multiplier = +this.multiplier.value;
var multiplicand = +this.multiplicand.value;
this.answer.value = multiplier * multiplicand;
return false; // keep the form from submitting
};
};​
})(window.onload);
Note i'm attaching the meat code to the form, rather than the button, because hitting Enter in either of the factor boxes will trigger a submit as well. You could still attach to the button if you wanted, and just add a submit handler that returns false. But IMO it's better this way -- that way the form works just the same with JS as without (assuming the script on the server fills in the boxes appropriately), except it won't require a round trip to the server.