Pass variables to JavaScript in ExpressJS

I am completely lost on this; I am using NodeJS to fetch a JSON and I need to pass the variable to my page and have JavaScript use the data.

app.get('/test', function(req, res) {
    res.render('testPage', {
        myVar: 'My Data'
    });

That is my Express code (very simple for testing purposes); now using EJS I want to gather this data which I know to render on the page is simply

<%= myVar %>

But I need to be able to gather this data in JavaScript (if possible within a .js file) but for now just to display the variable in an Alert box I have tried

In Jade it is like alert('!{myVar}') or !{JSON.stringify(myVar)}. Can I do something similar in EJS. I don't need any field like <input type=hidden> and taking the value of the field in javascript. If anyone can help be much appreciated

You could use this (client-side):

<script>
  var myVar = <%- JSON.stringify(myVar) %>;
</script>

You could also get EJS to render a .js file:

app.get('/test.js', function(req, res) {
  res.set('Content-Type', 'application/javascript');
  res.render('testPage', { myVar : ... });
});

However, the template file (testPage) would still need to have the .html extension, otherwise EJS won't find it (unless you tell Express otherwise).

As @ksloan points out in the comments: you do have to be careful what myVar contains. If it contains user-generated content, this may leave your site open for script injection attacks.

A possible solution to prevent this from happening:

<script>
  function htmlDecode(input){
    var e = document.createElement('div');
    e.innerHTML = input;
    return e.childNodes.length === 0 ? "" : e.childNodes[0].nodeValue;
  }
  var myVar = JSON.parse(htmlDecode("<%= JSON.stringify(myVar) %>"));
</script>

The main difficulty here is to avoid XSS risks if myVar contains quotes, or </script> for example. To avoid this problem, I propose to use Base64 encoding after JSON.stringify. This would avoid all risks related to quotes or HTML tags since Base64 only contains "safe" characters to put in a quoted string.

The solution I propose:

EJS file:

<script>
  var myVar = <%- passValue(myVar) %>
</script>

which will render into something like (for example here myVar = null):

<script>
  var myVar = JSON.parse(Base64.decode("bnVsbA=="))
</script>

Server-side NodeJS:

function passValue(value) {
  return 'JSON.parse(Base64.decode("' + new Buffer(JSON.stringify(value)).toString('base64') + '"))'
}

Client-side JS (this is an implementation of Base64 decoding that works with Unicode, you can use another if you prefer but be careful if it supports Unicode):

var Base64={_keyStr:"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",encode:function(e){var t="";var n,r,i,s,o,u,a;var f=0;e=Base64._utf8_encode(e);while(f<e.length){n=e.charCodeAt(f++);r=e.charCodeAt(f++);i=e.charCodeAt(f++);s=n>>2;o=(n&3)<<4|r>>4;u=(r&15)<<2|i>>6;a=i&63;if(isNaN(r)){u=a=64}else if(isNaN(i)){a=64}t=t+this._keyStr.charAt(s)+this._keyStr.charAt(o)+this._keyStr.charAt(u)+this._keyStr.charAt(a)}return t},decode:function(e){var t="";var n,r,i;var s,o,u,a;var f=0;e=e.replace(/[^A-Za-z0-9\+\/\=]/g,"");while(f<e.length){s=this._keyStr.indexOf(e.charAt(f++));o=this._keyStr.indexOf(e.charAt(f++));u=this._keyStr.indexOf(e.charAt(f++));a=this._keyStr.indexOf(e.charAt(f++));n=s<<2|o>>4;r=(o&15)<<4|u>>2;i=(u&3)<<6|a;t=t+String.fromCharCode(n);if(u!=64){t=t+String.fromCharCode(r)}if(a!=64){t=t+String.fromCharCode(i)}}t=Base64._utf8_decode(t);return t},_utf8_encode:function(e){e=e.replace(/\r\n/g,"\n");var t="";for(var n=0;n<e.length;n++){var r=e.charCodeAt(n);if(r<128){t+=String.fromCharCode(r)}else if(r>127&&r<2048){t+=String.fromCharCode(r>>6|192);t+=String.fromCharCode(r&63|128)}else{t+=String.fromCharCode(r>>12|224);t+=String.fromCharCode(r>>6&63|128);t+=String.fromCharCode(r&63|128)}}return t},_utf8_decode:function(e){var t="";var n=0;var r=c1=c2=0;while(n<e.length){r=e.charCodeAt(n);if(r<128){t+=String.fromCharCode(r);n++}else if(r>191&&r<224){c2=e.charCodeAt(n+1);t+=String.fromCharCode((r&31)<<6|c2&63);n+=2}else{c2=e.charCodeAt(n+1);c3=e.charCodeAt(n+2);t+=String.fromCharCode((r&15)<<12|(c2&63)<<6|c3&63);n+=3}}return t}}

if you have more complex objects like an array, you can do this :

<% if (myVar) { %>
   <script>
      myVar = JSON.parse('<%- JSON.stringify(myVar) %>');
   </script>
<% } %>

otherwise, previous solutions you have seen will not work

Pass variables to JavaScript in ExpressJS

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