A small little helper for creating a template tag where one can modify the static strings, the values and/or the final output in some way. Eg. trim some whitespaces?

template-tag-factory.js

/**
 * @template T
 * @param {T} value
 * @returns {T}
 */
const passthrough = value => value;

/**
 * @param {object} [options]
 * @param {(value: string) => string} [options.staticCallback]
 * @param {(value: string|number) => string} [options.valueCallback]
 * @param {(value: string) => string} [options.finalCallback]
 * @returns {(strings: TemplateStringsArray, ...values: (string|number)[]) => string}
 */
const templateTagWithModifier = (options = {}) => {
  if (!options || typeof options !== 'object') throw new TypeError('Expected argument to be an object');

  const { staticCallback, valueCallback, finalCallback } = options;

  if (staticCallback && typeof staticCallback !== 'function') throw new TypeError('Expected staticCallback to be a function');
  if (valueCallback && typeof valueCallback !== 'function') throw new TypeError('Expected valueCallback to be a function');
  if (finalCallback && typeof finalCallback !== 'function') throw new TypeError('Expected finalCallback to be a function');

  /**
   * @param {TemplateStringsArray} strings
   * @param {...string|number} values
   * @returns {string}
   */
  const func = (strings, ...values) => (finalCallback || passthrough)(
    strings.reduce(
      (result, value, i) =>
        result +
        (staticCallback ? staticCallback(value) : value) +
        (valueCallback ? valueCallback(values[i] || '') : (values[i] || '')),
      ''
    )
  );

  return func;
};

module.exports = templateTagWithModifier;