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How to set on change as a default on page load jquery?

I have created a on change method for a select box of my project. On selecting particular option it is basically showing and hiding a div which is perfectly working fine. Now, my problem is when first time page is loading this show and hide not working for first default section of form. Can I make this onchange function also working when page load first time.
$('.contact-form').on('change', (e) => {
var selectedId = $(e.currentTarget).val();
var listofforms = $("#discount").data("display-for").split(",");
if (listofforms.indexOf(selectedId) !== -1) {
$("#discount").collapse('show');
}
else {
$("#discount").collapse('hide');
}
});

Here you go with a solution
function changeMethod(selectedId) {
var listofforms = $("#discount").data("display-for").split(",");
if (listofforms.indexOf(selectedId) !== -1) {
$("#discount").collapse('show');
}
else {
$("#discount").collapse('hide');
}
}
changeMethod($('.contact-form').val())
$('.contact-form').on('change', (e) => {
changeMethod($(e.currentTarget).val());
});
You need to move your code outside the change event, so I have kept your existing code within a method changeMethod.
Then call the method from to places
From you change event method
OnLoad of the JS file

Is it possible can I make my on change trigger on page load
Yes, you will just need to change your on change event from e.currentTarget to this as on page load e.currentTarget will be null, but this always points to the current element like:
$('.contact-form').on('change', function() {
var selectedId = $(this).val();
// Your other logic here
});
and to trigger this change event on page load, simply add .change() at last like:
$('.contact-form').on('change', function() {
var selectedId = $(this).val();
// Your other logic here
}).change(); //<---- here

Javascript help-Menu

Want to make it so when my menu items transition away, the search bar pops up.
let menuItemsQuerySelector = document.querySelectorAll(".menu-item");
searchElement.addEventListener("click", function() {
console.log("Clicked search");
menuItemsQuerySelector.forEach(function(menuItem) {
console.log("Boom");
menuItem.classList.toggle("hide-item");
});
});
};
this is what i have so far to make the toggle animation work. my claases for the search bar are, search-from, i need to make it active somehow when the menu disappears. The css class is already set up.

You can use the "transitionend" event to execute code after the transition ends.
You would have to add a boolean to check whether the transition was hidden-visible or visible-hidden
let hidden = false;
searchElement.addEventListener("click", function() {
hidden = true;
//your other code
});
//Further down the line when showing your elements again
hidden = false;
However seeing that you have multiple elements that transition at the same time, you could either:
Hook the event only on one of them
menuItemsQuerySelector[0].on('transitionend', () => {
if(hidden)
//your code here
});
or Use a timed function
setTimeout(() => {
if(hidden)
//your code here
}, <delay in millisecods>);

How can I observe changes to my DOM and react to them with jQuery?

I have this function where I toggle a class on click, but also append HTML to an element, still based on that click.
The problem is that now, I'm not listening to any DOM changes at all, so, once I do my first click, yup, my content will be added, but if I click once again - the content gets added again, because as far as this instance of jQuery is aware, the element is not there.
Here's my code:
(function($) {
"use strict";
var closePluginsList = $('#go-back-to-setup-all');
var wrapper = $('.dynamic-container');
$('#install-selected-plugins, #go-back-to-setup-all').on('click', function(event) {
$('.setup-theme-container').toggleClass('plugins-list-enabled');
if ( !wrapper.has('.plugins-container') ){
var markup = generate_plugins_list_markup();
wrapper.append(markup);
} else {
$('.plugins-container').hide();
}
});
//Below here, there's a lot of code that gets put into the markup variable. It's just generating the HTML I'm adding.
})(jQuery);
Someone suggested using data attributes, but I've no idea how to make them work in this situation.
Any ideas?

You could just do something like adding a flag and check for it before adding your markup.
var flag = 0;
$('#install-selected-plugins, #go-back-to-setup-all').on('click', function(event) {
$('.setup-theme-container').toggleClass('plugins-list-enabled');
if ( !wrapper.has('.plugins-container') ){
var markup = generate_plugins_list_markup();
if(flag == 0){
wrapper.append(markup);
flag = 1;
}
} else {
$('.plugins-container').hide();
}
});

If you want to add element once only on click then you should make use of .one() and put logic you want to execute once only in that handler.
Example :
$(document).ready(function(){
$("p").one("click", function(){
//this will get execute once only
$(this).animate({fontSize: "+=6px"});
});
$("p").on("click", function(){
//this get execute multiple times
alert('test');
});
});
html
<p>Click any p element to increase its text size. The event will only trigger once for each p element.</p>

How Store and Disable Event of another element Temporary

I am looking for a way to manage the events. I have a hover function for element A, and click function for element B. I want to disable A`s hover function temporary while the second click of B.
I am looking for a way that not necessary to rewrite the hole function of A inside of B. Something very simply just like "Store and Disable Event, Call Stored Function"
I found some technique like .data('events') and console.log. I tired but failed, or maybe I wrote them in a wrong way.
Please help and advice!
$(A).hover();
$(b).click(
if($.hasData($(A)[0])){ // if A has event,
//STORE all the event A has, and disable
}else{
//ENABLE the stored event for A
}
);

Try this
var hoverme = function() {
alert('Hover Event Fired');
};
$('.A').hover(hoverme);
var i = 0;
$('.B').on('click', function(){
if(i%2 === 0){
// Unbind event
$('.A').off('hover');
}
else{
// Else bind the event
$('.A').hover(hoverme);
}
i++;
});
Check Fiddle

I think that what you want to do is something like this (example for JQuery 1.7.2):
$("#a").hover(function(){alert("test")});
$("#a")[0].active=true;
$("#b").click(function(){
if($("#a")[0].active){
$("#a")[0].storedEvents = [];
var hoverEvents = $("#a").data("events").mouseover;
jQuery.each(hoverEvents , function(key,handlerObj) {
$("#a")[0].storedEvents.push(handlerObj.handler);
});
$("#a").off('hover');
}else{
for(var i=0;i<$("#a")[0].storedEvents.length;i++){
$("#a").hover($("#a")[0].storedEvents[i]);
}
}
$("#a")[0].active = ($("#a")[0].active)==false;
});​
JSFiddle Example
But there are a couple of things that you must have in consideration:
This will only work if you add the events with JQuery, because JQuery keeps an internal track of the event handlers that have been added.
Each version of JQuery handles data("events") differently, that means that this code may not work with other version of JQuery.
I hope that this helps.
EDIT:
data("events") was an internal undocumented data structure used in JQuery 1.6 and JQUery 1.7, but it has been removed in JQuery 1.8. So in JQuery 1.8 the only way to access the events data is through: $._data(element, "events"). But keep in mind the advice from the JQuery documentation: this is not a supported public interface; the actual data structures may change incompatibly from version to version.

You could try having a variable that is outside the scope of functions a and b, and use that variable to trigger the action to take in function b on function a.
var state;
var a = function() {
if(!state) {
state = true;
// Add hover action and other prep. I'd create a third function to handle this.
console.log(state);
};
var b = function() {
if(state) {
state = false;
// Do unbinding of hover code with third function.
} else {
state = true;
// Do whatever else you needed to do
}
}
Without knowing more about what you're trying to do, I'd try something similar to this.

It sounds like you want to disable the click hover event for A if B is clicked.
$("body").on("hover", "#a", function(){
alert("hovering");
});
$("#b").click( function(){
$("body").off("hover", "#a", function() {
alert("removed hovering");
});
});
You can use the jQuery off method, have a look at this fiddle. http://jsfiddle.net/nKLwK/1/

Define a function to assign to hover on A element, so in b click, call unbind('hover') for A element and in second click on b element define again a function to hover, like this:
function aHover(eventObject) {
// Todo when the mouse enter object. You can use $(this) here
}
function aHoverOut(eventObject) {
// Todo when the mouse leave the object. You can use $(this) here
}
$(A).hover(aHover, aHoverOut);
// ...
$(b).click(function(eventObject) {
if($.hasData($(A)[0])){ // if A has event,
$(A).unbind('mouseenter mouseleave'); // This is because not a event hover, jQuery convert the element.hover(hoverIn, hoverOut) in element.bind('mouseenter', hoverIn) and element.bind('mouseleave', hoverOut)
}else{
$(A).hover(aHover, aHoverOut);
}
});

There are provably better ways to do it, but this works fine, on document ready do this:
$("#a")[0].active=false;
$("#b").click(function(){
$("#a")[0].active = ($("#a")[0].active)==false;
if($("#a")[0].active){
$("#a").hover(function(){alert("test")});
}else{
$("#a").off('hover');
}
});
JSFiddle example

You can use .off function from jQuery to unbind the hover on your "a" element.
function hoverA() {
alert('I\'m on hover');
}
$('#a').hover( hoverA );
var active = true;
$('#b').on('click', function(){
if(active){
$('#a').off('hover');
active = false;
} else{
$('#a').hover(hoverA);
active = true;
}
});
Live demo available here : http://codepen.io/joe/pen/wblpC

How to call a function with jQuery blur UNLESS clicking on a link?

I have a small jQuery script:
$('.field').blur(function() {
$(this).next().children().hide();
});
The children that is hidden contains some links. This makes it impossible to click the links (because they get hidden). What is an appropriate solution to this?
This is as close as I have got:
$('.field').blur(function() {
$('*').not('.adress').click(function(e) {
foo = $(this).data('events').click;
if(foo.length <= 1) {
// $(this).next('.spacer').children().removeClass("visible");
}
$(this).unbind(e);
});
});
The uncommented line is suppose to refer to the field that is blurred, but it doesn't seem to work. Any suggestions?

You can give it a slight delay, like this:
$('.field').blur(function() {
var kids = $(this).next().children();
setTimeout(function() { kids.hide(); }, 10);
});
This gives you time to click before those child links go away.

This is how I ended up doing it:
var curFocus;
$(document).delegate('*','mousedown', function(){
if ((this != curFocus) && // don't bother if this was the previous active element
($(curFocus).is('.field')) && // if it was a .field that was blurred
!($(this).is('.adress'))
) {
$('.' + $(curFocus).attr("id")).removeClass("visible"); // take action based on the blurred element
}
curFocus = this; // log the newly focussed element for the next event
});

I believe you can use .not('a') in this situation:
$('.field').not('a').blur(function() {
$(this).next().children().hide();
});
This isn't tested, so I am not sure if this will work or not.