Efficient Javascript String Replacement

You could probably adapt this code to do what you want:

let user = {
    "firstName": "John",
    "login": "john_doe",
    "password": "test",
};

let template = `Hey {firstName},
    
    You recently requested your password.
    login: {login}
    password: {password}
    
    If you did not request your password, please disregard this message.
    `;

template = template.replace(/{([^{}]+)}/g, function(keyExpr, key) {
    return user[key] || "";
});

You might also want to look into JavaScriptTemplates

It looks like you want to use a template.

//Updated 28 October 2011: Now allows 0, NaN, false, null and undefined in output. 
function template( templateid, data ){
    return document.getElementById( templateid ).innerHTML
      .replace(
        /%(\w*)%/g, // or /{(\w*)}/g for "{this} instead of %this%"
        function( m, key ){
          return data.hasOwnProperty( key ) ? data[ key ] : "";
        }
      );
}

Explanation of the code:

• Expects templateid to be the id of an existing element.
• Expects data to be an object with the data.
• Uses two parameters to replace to do the substitution:
• The first is a regexp that searches for all %keys% (or {keys} if you use the alternate version). The key can be a combination of A-Z, a-z, 0-9 and underscore _.
• The second is a anonymous function that gets called for every match.
• The anonymous function searches the data object for the key that the regexp found. If the key is found in the data, then the value of the key is returned and that value will be replacing the key in the final output. If the key isn't found, an empty string is returned.

Example of template:

<div id="mytemplate">
  <p>%test%</p>
  <p>%word%</p>
</div>

Example of call:

document.getElementById("my").innerHTML=template("mytemplate",{test:"MYTEST",word:"MYWORD"});