Concept for source-mapping the output of a Handlebars-template back to the template

example.js

var Handlebars = require('handlebars');
var LineCounter = require("line-counter");
var env = Handlebars.create();

function SourceMapCompiler() {
}

SourceMapCompiler.prototype = new Handlebars.JavaScriptCompiler();

SourceMapCompiler.prototype.appendToBuffer = function (source, location, explicit) {
  var result = Handlebars.JavaScriptCompiler.prototype.appendToBuffer.apply(this, [source, location, true])
  return [`helpers.$consumeSourceMapEntry(${result[1].line}, ${result[1].column}, buffer.length);`, result]
}

env.JavaScriptCompiler = SourceMapCompiler
var sourceMappings = []
// A helper seems to be about the only way to pass data from the template function to the calling context
env.registerHelper("$consumeSourceMapEntry", function (sourceLine, sourceColumn, targetIndex) {
    sourceMappings.push({
      source: {
        column: sourceColumn+1,
        line: sourceLine
      },
      target: targetIndex
    })
  }
)

const template = '1{{abc}}\n2a\n3{{cde}}\n4\n5b\n6\n\n\n\n\n{{abc}}';
var output = env.compile(template)({
  abc: 'a\nmultiline\string',
  cde: 'x'
});

// Map target-indexes to lines/columns
var counter = new LineCounter(output);
sourceMappings.forEach(function(entry) {
  entry.target = counter.locate(entry.target)
})

console.log(sourceMappings)
console.log(JSON.stringify({template, output},null,2))

This is a proof-of-concept. The source-maps that the Handlebars project generates are just from compiled-template to the template (as far as I know). For debugging and developing templates, what I sometimes need is from "template-output" to "template".
This is an idea of how to achieve it, without changing Handlebars itself.

As a workaround, the mappings are passed back to the callee through a helper. This should actually be a hook inside Handlebars, but I have to think about it.

Mhm, this does not work with partials and block-helpers:

• The buffer is new when sub-programs are executed. Somehow I have to grab the point where the partial is called and recalculated the target-index based on that point.
• The SourceNodes in the compile function do not have the information in which partial they occur.