Javascript string interpolation not working from text input

Question

I am trying to implement string interpolation by letting user provide template from the form.

Consider these inputs:

• Input with id mapping_template
• Input with id key
• Input with id value

When users type following in inputs:

1. mapping_template has value ${key} = ${value}
2. key has value a key
3. value has value the value

This is my function, I invoke it like so interpolateMappingTemplate (mapping_template, 'a key', 'the value') :

var interpolateMappingTemplate = function(mappingTemplate, key, value) {
  return `${templateString}`;
}

I was expecting when I interpolate template to get this a key = the value but instead my function outputs ${key} = ${value}.

What am I doing wrong here?