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PHP logic to Javascript front into html tags

I am currently trying to convert a lot of backend code to front end (to lighten the load on a small system).
The code at the moment calls a PHP function to return specific information. (e.g. image locations, strings, styling)
I am converting this code to its js equivalent, the content from Mysql was converted to JSON and stored in a read only file and I am accessing that file using this code:
<script>
function jsread(tag) {
$.getJSON("/strings.json", function(result){
document.write(result[tag]['value']);
});
}
</script>
I want the function to "print" where ever it is invoked. document write writes the value to the page but stops all other loading and write only the value.
Let me be very clear on this: I DO NOT want to use anything that needs extra calls or references out side of this function, that will take months of work so no getting elements by their IDs I have already view many questions on this subject and none are what I can work with. I need something that can be applied to every situation. Other wise I will just have to read the JSON using PHP as a middle compromise.

The problem here is, document.write()'s behaviour is crazy across all the browsers, because, it directly modifies the document object and messes up with the events attached. So it is always better to avoid this function as each browser defines it differently and has a different effect on the same code, with different browsers.
Is there a way to use them without a direct reference?
Solution
The wise thing is, as I said in the comments, it is better to use one of the jQuery functions safely, which create a textNode and insert it the right way, without affecting the others:
<script>
function jsread(tag) {
$.getJSON("/strings.json", function(result){
$("body").append(result[tag]['value']);
});
}
</script>
In case, if you wanna do something like having a placeholder and doing stuff, then you can try giving something like this:
$(function () {
var data = "Dummy Data, that would probably get returned from the getJSON";
// Inside the Success function, do this:
$("span.placeholder-of-the-json").replaceWith(data);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<span class="placeholder-of-the-json">This gets replaced</span>

Better JavaScript Organisation and Execution The Unobtrusive Way - Self-Executing Anonymous Func

I'm slowly getting a better understanding of JavaScript but I'm stuck on how best to tackle this particular organization/execution scenario.
I come from a C# background and am used to working with namespaces so I've been reading up on how to achieve this with JavaScript. I've taken what was already starting to become a large JavaScript file and split it out into more logical parts.
I've decided on a single file per page for page specific JavaScript with anything common to two or more pages, like reusable utility functions, in another namespace and file.
This makes sense to me at the moment and seems to be a popular choice, at least during the development process. I'm going to use a bundling tool to combine these disparate files for deployment to production anyway so anything that makes development more logical and easier to find code the better.
As a result of my inexperience in dealing with lots of custom JavaScript I had a function defined in the common JavaScript file like this:
common.js
$(document).ready(function () {
var historyUrl = '/history/GetHistory/';
$.getJSON(historyUrl, null, function (data) {
$.each(data, function (index, d) {
$('#history-list').append('<li>' + d.Text + '</li>');
});
});
});
This is obviously far from ideal as it is specific to a single page in the application but was being executed on every page request which is utterly pointless and insanely inefficient if not outright stupid. So that led me to start reading up on namespaces first.
After a bit of a read I have now moved this to a page specific file and re-written it like this:
Moved from common.js to historyPage.js
(function(historyPage, $, undefined) {
historyPage.GetHistory = function () {
var historyUrl = '/history/GetHistory/';
$.getJSON(historyUrl, null, function (data) {
$.each(data, function (index, d) {
$('#history-list').append('<li>' + d.Text + '</li>');
});
});
};
}( window.historyPage = window.historyPage || {}, jQuery ));
I found this pattern on the jQuery Enterprise page. I'm not going to pretend to fully understand it yet but it seems to be a very popular and the most flexible way of organizing and executing JavaScript with various different scopes whist keeping things out of the global scope.
However what I'm now struggling with is how to properly make use of this pattern from an execution point of view. I'm also trying to keep any JavaScript out of my HTML Razor views and work in an unobtrusive way.
So how would I now call the historyPage.GetHistory function only when it should actually execute ie: only when a user navigates to the History page on the web site and the results of the function are required?

From looking at the code, it would seem that the easiest test would be to check if the page you are on contains an element with an id of history-list. Something like this:
var $histList = $('#history-list');
if($histList.length > 0){
// EXECUTE THE CODE
}
Though if it really only ever needs to run on one given page, maybe it's just not a good candidate for a shared javascript file.

Using the code I have detailed above in the question I have gotten it working by doing the following:
In _Layout.cshtml
#if (IsSectionDefined("History"))
{
<script type="text/javascript">
$(document).ready(function () {
#RenderSection("History", required: false)
});
</script>
}
In History.cshtml
#section History
{
historyPage.GetHistory();
}
The code is executing as required only when the user requests the History page on the web site. Although the comment from #Dagg Nabbit above has thrown me a curve ball in that I thought I was on the right track ... Hmm ...

javascript strategy / best practice

I'm getting into writing some more complex javascript applications, and I'm running into the limitations of my own knowledge-- please forgive any naming errors or obvious noob stuff, I'm not a js pro!
I have about 4 or 5 scripts I've put in their own files, just to keep things a little easier to maintain. So maybe there's one script that deals with building page elements (like complex forms), another that just handles data, creating generic ajax request objects, defining parsers and error functions for the data returned, and another that is purely display-oriented.
I've set global variables in the page that then get populated by various scripts that get loaded at run time. For example, I define var myapp = { }; in the main HTML page, and then in the scripts various function populate this "namespace" like:
myapp.myfunction = function(){
// do stuff
}
The problem is that despite all the scripts including a $(document).ready(function() block that wraps all function definitions, when a function is called from one script that refers to another (that is, if my data.js file calls a function myapp.myDisplayFunction that is in the display.js file, I sometimes get an Object has no method 'myDisplayFunction'
Other than slamming all functions into one massive script, how do you deal with this problem? Is there a best practice that I'm missing? or is this just a question of specifying a different order that the scripts are called in?
Thanks

When you are not sure if method you are about to call exists (is already loaded) you can do a check:
if (myapp) //my app namespace is defined
{
if (myapp.myFunction) //myFunction is defined
{
myapp.myFunction();
}
else
alert('You have to load myFile.js first!');
}

Just check for the function before using:
if(typeof(myapp.myDisplayFunction) !== undefined) {
// do your stuff
} else {
// wait for a while
}
And check whether you have async attribute set while loading the .js files.

How to do per-page javascript with the Rails asset pipeline

I understand that for performance reasons it is better to let the asset pipeline concatenate and minify all my javascript and send the whole lot with every page request. That's fair enough
However, a bunch of my javascript is things like binding specific behaviours to specific page elements - stuff like
$('button').click(function(e) { $('input.sel').val(this.name); }
and I would feel more comfortable if I knew that this code was being executed only on that page - not on evey other page which might coincidentally have elements with the same IDs or which matched the same selectors How do people deal with this?
I would rather not put all this stuff inline in elements, just because when it gets to be more than about two lines long, keeping javascript correctly indented inside an .html.erb file is more work than it needs to be

Here is what I do (based on some stackoverflow answers):
application_helper.rb
def body_page_name
[controller_name.classify.pluralize, action_name.classify].join
end
application.html.haml
%body{data: {page: body_page_name}}
application.js
$(function() {
var page = $("body").data("page");
if("object" === typeof window[page])
window[page].init();
});
And in appropriate js file there's an object called ControllerAction:
tickets.js
var TicketsShow = new function() {
var self = this;
self.init = function() {
// code which may call other functions in self
};
};
There's probably better way to do it, but this works for me

I'll describe what I currently do, just in case it gives anyone a better idea
1) I changed the 'body' tag in my application.html.erb to add the current controller and action as data- attributes
<body data-controller="<%= controller.controller_name %>"
data-action="<%= controller.action_name %>" >
2) I test this at the top of the relevant javascript
$(document).ready(function() {
if($('body').data('controller')=='stories') {
$('.story').click(function(e) {
var u=$(this).data('url');
u && (document.location=u);
});
}
});
I can't decide if I think this is a good idea or not

For page specific JavaScript, I typically do something like this:
Application Helper
In the application helper I create a class attribute (though you could just as well use a data attribute instead).
module ApplicationHelper
def body_attributes
controller = params[:controller].gsub('/', ' ')
action = params[:action]
version = #version ? "version_#{#version}" : nil
{
class: ([controller, action, version] - [nil]).join(' ')
}
end
end
Note I'm also adding a version string. This helps with Google content experiments, and makes A/B testing a breeze.
Application.html.haml
In my global layout file, I do something like this to insert the attributes on the body tag:
!!! 5
%html
%head
...
%body{body_attributes}
script.js
Now in my page specific script, I just check for the class attributes, like this:
$(function () {
if ($('body.pledge.new, body.pledge.create').length > 0) {
// do work here...
}
});
The advantage of this method is that getting the body by class is very quick. The script inside the conditional will not be executed at all on any page apart than the ones I choose, so minimal overhead, and I don't need to change my selectors throughout the code.
EDIT
Note that this answer is now 3 years old. You should be using client-side routing with a framework like React instead.

I'd add a class to the BODY tag, allowing you to identify each page, and therefore each control per page.
<body class='page1'>
JS:
$('.page1 button').click(function(e) { $('input.sel').val(this.name); }

I've done it and seen it done in several different ways:
Rigging up the mvc to be able to load a particular js file per page, named along the same lines as a controller file. Like: <controller-name>.js
Making a url parser in JS and then setting a global variable to the current page: UrlParams.currentView = 'dashboard'; and then saying if(UrlParams.currentView == 'dashboard') { //do specific js here }
Setting a unique identifier as the page class or ID and then targeting that with your JS selectors. $('#dashboard').xyz();

send data to JS file without using global variables

I need to send data in a HTML page to a script file that is loaded in that page. The simplest way i can think of is to use a global variable which is defined in the page and accessed in the script file.
We all know global state is bad, so i started thinking about the options available for passing data from HTML page to script file without using global state. I cant find (or think of) any.
I am curious whether this is possible. Any ideas?

It really depends what you're doing. In general, I wouldn't advise this methodology, but it's something to consider depending on your circumstances. For the sake of this example, I'll assume you're using jQuery (if not, replace the document.ready with whatever you want to use for onDOMReadyStateChange monitoring).
In the HTML:
<script type='text/json-data' id='some_data_set'>
{ 'foo': 'bar', 'baz': 1 }
</script>
In the JavaScript:
$(function() {
var myData = JSON.parse($('script#some_data_set').html());
// YOUR CODE GOES HERE
});

Nope. All the javascript scope starts from a global level, therefore you must have at least one global reference to your data.
Let's say you wanted to store a list of products and events:
var myGlobalData = { "products":<products>, "events":<events> };
Where <products> and <events> are two different data blocks.
If you're paranoid on global objects, you can simply delete the reference point (thus it's contents) after you finished using it, as follows:
delete window.myGlobalData;

One option is to scope your data. For example, in JS file you can define an object like:
var processor = {
function setData(o) { // do stuff
}
};
Then in your HTML you know that the data is scoped to the processor. So you can do something like:
processor.setData({someData});