Here is a function that can be used to replace JavaScript variable name (object property, or array element) from string with its associated value defined in JavaScript, using RegExp.
// Function that replace token from string with equivalent variable value
// Receives the string-template. Returns that string with variable names replaced
function replaceStrVar(str){
 // JavaScript & jQuery Courses - https://coursesweb.net/javascript/
  str = str.replace(/%(.*?)%/gi, function(a,b) {
    // if token is an array item, else, is object property, or variable
    if(b.match(/[a-z0-9_]+\[[a-z0-9_]+\]/i)) {
      var arritm = b.match(/([a-z0-9_]+)\[([a-z0-9_]+)\]/i);      // gets an array with the matched items
      return window[arritm[1]][arritm[2]];
    }
    else {
      var voitm = b.split('.');
      return (voitm.length == 2) ? window[voitm[0]][voitm[1]] : window[voitm[0]];
    }
  });

  return str;
}
- The names for variable /array /object.property in the string must be added within %...% .

This function can be used to create a simple template-string in JavaScript.

Here's an example of replaceStrVar() function usage, using a variable, an array, and object, and a string (template) with their names that will be replaced with their values and the result is added into a HTML element.
<div id="testpl">String Template:<br/><span class="sbi">The website: &lt;b&gt;%website%&lt;/b&gt; contains: %obj.content1%, and it has around &lt;b&gt;%visits[2]%&lt;/b&gt; uniques visitors /day.</span></div>
<button onclick="testpl()">Test replaceStrVar()</button>

<script type="text/javascript"><!--
// Function that replace token from string with equivalent variable value
function replaceStrVar(str){
 // JavaScript & jQuery Courses - https://coursesweb.net/javascript/
  str = str.replace(/%(.*?)%/gi, function(a,b) {
    // if token is an array item, else, is object property, or variable
    if(b.match(/[a-z0-9_]+\[[a-z0-9_]+\]/i)) {
      var arritm = b.match(/([a-z0-9_]+)\[([a-z0-9_]+)\]/i);      // gets an array with the matched items
      return window[arritm[1]][arritm[2]];
    }
    else {
      var voitm = b.split('.');
      return (voitm.length == 2) ? window[voitm[0]][voitm[1]] : window[voitm[0]];
    }
  });

  return str;
}

  /* Test replaceStrVar() function */

var website = 'https://coursesweb.net';      // simple variable
var visits = new Array(100, 1000, 2000);        // array

// object
var obj = new Object();
 obj.content1 = 'Web Programming Courses';
 obj.content2 = 'Web Development Tutorials';

// The string (template)
var tpl = 'The website: <b>%website%</b> contains: %obj.content1%, and it has around <b>%visits[2]%</b> uniques visitors /day.';

// adds the replacement in #testpl HTML element
function testpl(){
  document.getElementById('testpl').innerHTML = replaceStrVar(tpl);
}
//-->
</script>
Results:
String Template:
The website: <b>%website%</b> contains: %obj.content1%, and it has around <b>%visits[2]%</b> uniques visitors /day.
- To see the result, click ->

Daily Test with Code Example

HTML
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function() filetype() extension_loaded()
if(extension_loaded("PDO") === true) echo "PDO is available."
Replace JavaScript variable name from string with its value

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