Typescript transpile es6 .js dependency to es5

The only thing that comes to my mind is hooking into the compilation process and transforming your dependencies before they get processed by TypeScript. This requires TypeScript transformers.

A transformer is a function to which the AST of your program is exposed. A basic example:

import * as ts from 'typescript';

export default (program: ts.Program) => {
    return (ctx: ts.TransformationContext) => {
        return (sourceFile: ts.SourceFile) => {
            function visitor(node: ts.Node): ts.Node {
                /**
                 * If that's the kind of node you were looking for,
                 * do something with it and return it. Otherwise:
                 */
                return ts.visitEachChild(node, visitor, ctx);
            }

            return ts.visitEachChild(sourceFile, visitor, ctx);
        };
    };
}

If you are using Webpack, you could plug it into your build pipeline in your Webpack configuration file.

webpack.config.js

const transformer = require('./your-custom-transformer');

module.exports = {
  /* ... */
  module: {
    rules: [
      {
        test: /\.ts$/,
        loader: 'ts-loader', // (or 'awesome-typescript-loader')
        options: {
          getCustomTransformers: program => ({
            before: [
              transformer(program)
            ]
          })
        }
      }
    ]
  }
};

Unfortunately, there isn't a way to do convert dependencies from ES6 to ES5. That option in tsconfig.json only affects how TypeScript code is transpiled. What you should do is use an ES5 version of your helper-from-node-modules. For example, Angular is distributed with several packages, for ES5 (umd), ES5, ES6 ... Then, in the package.json of the library there are options to tell the packager (usually webpack) what version to use, depending on the target use for TypeScript.

If the library you're using does not support that, the only option you have is to transpile it to ES5 yourself, maybe using babel, or use an alternative. Is is strange, however, for a library to be only distributed as ES6.