Passing parameters to JavaScript files

Often I will have a JavaScript file that I want to use which requires certain variables be defined in my web page.

So the code is something like this:

<script type="text/javascript" src="file.js"></script>
<script type="text/javascript">
   var obj1 = "somevalue";
</script>

But what I want to do is:

<script type="text/javascript" 
     src="file.js?obj1=somevalue&obj2=someothervalue"></script>

I tried different methods and the best one yet is to parse the query string like this:

var scriptSrc = document.getElementById("myscript").src.toLowerCase();

And then search for my values.

I wonder if there is another way to do this without building a function to parse my string.

Do you all know other methods?

Answers:

Answer

I'd recommend not using global variables if possible. Use a namespace and OOP to pass your arguments through to an object.

This code belongs in file.js:

var MYLIBRARY = MYLIBRARY || (function(){
    var _args = {}; // private

    return {
        init : function(Args) {
            _args = Args;
            // some other initialising
        },
        helloWorld : function() {
            alert('Hello World! -' + _args[0]);
        }
    };
}());

And in your html file:

<script type="text/javascript" src="file.js"></script>
<script type="text/javascript">
   MYLIBRARY.init(["somevalue", 1, "controlId"]);
   MYLIBRARY.helloWorld();
</script>
Answer

You can pass parameters with arbitrary attributes. This works in all recent browsers.

<script type="text/javascript" data-my_var_1="some_val_1" data-my_var_2="some_val_2" src="/js/somefile.js"></script>

Inside somefile.js you can get passed variables values this way:

........

var this_js_script = $('script[src*=somefile]'); // or better regexp to get the file name..

var my_var_1 = this_js_script.attr('data-my_var_1');   
if (typeof my_var_1 === "undefined" ) {
   var my_var_1 = 'some_default_value';
}
alert(my_var_1); // to view the variable value

var my_var_2 = this_js_script.attr('data-my_var_2');   
if (typeof my_var_2 === "undefined" ) {
   var my_var_2 = 'some_default_value';
}
alert(my_var_2); // to view the variable value

...etc...

Answer

Another idea I came across was assigning an id to the <script> element and passing the arguments as data-* attributes. The resulting <script> tag would look something like this:

<script id="helper" data-name="helper" src="helper.js"></script>

The script could then use the id to programmatically locate itself and parse the arguments. Given the previous <script> tag, the name could be retrieved like this:

var name = document.getElementById("helper").getAttribute("data-name");

We get name = helper

Answer

Check out this URL. It is working perfectly for the requirement.

http://feather.elektrum.org/book/src.html

Thanks a lot to the author. For quick reference I pasted the main logic below:

var scripts = document.getElementsByTagName('script');
var myScript = scripts[ scripts.length - 1 ];

var queryString = myScript.src.replace(/^[^\?]+\??/,'');

var params = parseQuery( queryString );

function parseQuery ( query ) {
  var Params = new Object ();
  if ( ! query ) return Params; // return empty object
  var Pairs = query.split(/[;&]/);
  for ( var i = 0; i < Pairs.length; i++ ) {
    var KeyVal = Pairs[i].split('=');
    if ( ! KeyVal || KeyVal.length != 2 ) continue;
    var key = unescape( KeyVal[0] );
    var val = unescape( KeyVal[1] );
    val = val.replace(/\+/g, ' ');
    Params[key] = val;
  }
  return Params;
}
Answer

You use Global variables :-D.

Like this:

<script type="text/javascript">
   var obj1 = "somevalue";
   var obj2 = "someothervalue";
</script>
<script type="text/javascript" src="file.js"></script">

The JavaScript code in 'file.js' can access to obj1 and obj2 without problem.

EDIT Just want to add that if 'file.js' wants to check if obj1 and obj2 have even been declared you can use the following function.

function IsDefined($Name) {
    return (window[$Name] != undefined);
}

Hope this helps.

Answer

Here is a very rushed proof of concept.

I'm sure there are at least 2 places where there can be improvements, and I'm also sure that this would not survive long in the wild. Any feedback to make it more presentable or usable is welcome.

The key is setting an id for your script element. The only catch is that this means you can only call the script once since it looks for that ID to pull the query string. This could be fixed if, instead, the script loops through all query elements to see if any of them point to it, and if so, uses the last instance of such an script element. Anyway, on with the code:

Script being called:

window.onload = function() {
//Notice that both possible parameters are pre-defined.
//Which is probably not required if using proper object notation
//in query string, or if variable-variables are possible in js.
var header;
var text;

//script gets the src attribute based on ID of page's script element:
var requestURL = document.getElementById("myScript").getAttribute("src");

//next use substring() to get querystring part of src
var queryString = requestURL.substring(requestURL.indexOf("?") + 1, requestURL.length);

//Next split the querystring into array
var params = queryString.split("&");

//Next loop through params
for(var i = 0; i < params.length; i++){
 var name  = params[i].substring(0,params[i].indexOf("="));
 var value = params[i].substring(params[i].indexOf("=") + 1, params[i].length);

    //Test if value is a number. If not, wrap value with quotes:
    if(isNaN(parseInt(value))) {
  params[i] = params[i].replace(value, "'" + value + "'");
 }

    // Finally, use eval to set values of pre-defined variables:
 eval(params[i]);
}

//Output to test that it worked:
document.getElementById("docTitle").innerHTML = header;
document.getElementById("docText").innerHTML = text;
};

Script called via following page:

<script id="myScript" type="text/javascript" 
        src="test.js?header=Test Page&text=This Works"></script>

<h1 id="docTitle"></h1>
<p id="docText"></p>
Answer

might be very simple

for example

<script src="js/myscript.js?id=123"></script>
<script>
    var queryString = $("script[src*='js/myscript.js']").attr('src').split('?')[1];
</script>

You can then convert query string into json like below

var json = $.parseJSON('{"' 
            + queryString.replace(/&/g, '","').replace(/=/g, '":"') 
            + '"}');

and then can use like

console.log(json.id);
Answer

This can be easily done if you are using some Javascript framework like jQuery. Like so,

var x = $('script:first').attr('src'); //Fetch the source in the first script tag
var params = x.split('?')[1]; //Get the params

Now you can use these params by splitting as your variable parameters.

The same process can be done without any framework but will take some more lines of code.

Answer

Well, you could have the javascript file being built by any of the scripting languages, injecting your variables into the file on every request. You would have to tell your webserver to not dish out js-files statically (using mod_rewrite would suffice).

Be aware though that you lose any caching of these js-files as they are altered constantly.

Bye.

Answer

Nice question and creative answers but my suggetion is to make your methods paramterized and that should solve all your problems without any tricks.

if you have function:

function A()
{
    var val = external_value_from_query_string_or_global_param;
}

you can change this to:

function B(function_param)
{
    var val = function_param;
}

I think this is most natural approach, you don't need to crate extra documentation about 'file parameters' and you receive the same. This specially useful if you allow other developers to use your js file.

Answer

No, you cant really do this by adding variables to the querystring portion of the JS file URL. If its writing the portion of code to parse the string that bothers you, perhaps another way would be to json encode your variables and put them in something like the rel attribute of the tag? I don't know how valid this is in terms of HTML validation, if thats something you're very worried about. Then you just need to find the rel attribute of the script and then json_decode that.

eg

<script type='text/javascript' src='file.js' rel='{"myvar":"somevalue","anothervar":"anothervalue"}'></script>
Answer

It's not valid html (I don't think) but it seems to work if you create a custom attribute for the script tag in your webpage:

<script id="myScript" myCustomAttribute="some value" ....>

Then access the custom attribute in the javascript:

var myVar = document.getElementById( "myScript" ).getAttribute( "myCustomAttribute" );

Not sure if this is better or worse than parsing the script source string.

Answer

If you need a way that passes CSP check (which prohibits unsafe-inline) then you have to use nonce method to add a unique value to both the script and the CSP directive or write your values into the html and read them again.

Nonce method for express.js:

const uuidv4 = require('uuid/v4')

app.use(function (req, res, next) {
  res.locals.nonce = uuidv4()
  next()
})

app.use(csp({
  directives: {
    scriptSrc: [
      "'self'",
      (req, res) => `'nonce-${res.locals.nonce}'`  // 'nonce-614d9122-d5b0-4760-aecf-3a5d17cf0ac9'
    ]
  }
}))

app.use(function (req, res) {
  res.end(`<script nonce="${res.locals.nonce}">alert(1 + 1);</script>`)
})

or write values to html method. in this case using Jquery:

<div id="account" data-email="{{user.email}}"></div>
...


$(document).ready(() => {
    globalThis.EMAIL = $('#account').data('email');
}
Answer

Although this question has been asked a while ago, it is still relevant as of today. This is not a trivial approach using script file params, but I already had some extreme use-cases that this way was most suited.

I came across this post to find out a better solution than I wrote a while ago, with hope to find maybe a native feature or something similar.
I will share my solution, up until a better one will be implemented. This works on most modern browsers, maybe even on older ones, didn't try.

All the solutions above, are based on the fact that it has to be injected with predefined and well marked SCRIPT tag and rely completely on the HTML implementation. But, what if the script is injected dynamically, or even worse, what if you are write a library, that will be used in a variety of websites?
In these and some other cases, all the above answers are not sufficient and even becoming too complicated.

First, let's try to understand what do we need to achieve here. All we need to do is to get the URL of the script itself, from there it's a piece of cake.

There is actually a nice trick to get the script URL from the script itself. One of the functionalities of the native Error class, is the ability to provide a stack trace of the "problematic location", including the exact file trace to the last call. In order to achieve this, I will use the stack property of the Error instance, that once created, will give the full stack trace.

Here is how the magic works:

// The pattern to split each row in the stack trace string
const STACK_TRACE_SPLIT_PATTERN = /(?:Error)?\n(?:\s*at\s+)?/;
// For browsers, like Chrome, IE, Edge and more.
const STACK_TRACE_ROW_PATTERN1 = /^.+?\s\((.+?):\d+:\d+\)$/;
// For browsers, like Firefox, Safari, some variants of Chrome and maybe other browsers.
const STACK_TRACE_ROW_PATTERN2 = /^(?:.*[email protected])?(.*?):\d+(?::\d+)?$/;

const getFileParams = () => {
    const stack = new Error().stack;
    const row = stack.split(STACK_TRACE_SPLIT_PATTERN, 2)[1];
    const [, url] = row.match(STACK_TRACE_ROW_PATTERN1) || row.match(STACK_TRACE_ROW_PATTERN2) || [];
    if (!url) {
        console.warn("Something went wrong. You should debug it and find out why.");
        return;
    }
    try {
        const urlObj = new URL(url);
        return urlObj.searchParams; // This feature doesn't exists in IE, in this case you should use urlObj.search and handle the query parsing by yourself.
    } catch (e) {
        console.warn(`The URL '${url}' is not valid.`);
    }
}

Now, in any case of script call, like in the OP case:

<script type="text/javascript" src="file.js?obj1=somevalue&obj2=someothervalue"></script>

In the file.js script, you can now do:

const params = getFileParams();
console.log(params.get('obj2'));
// Prints: someothervalue

This will also work with RequireJS and other dynamically injected file scripts.

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