Type Safe Templates

template_string.ts

type Delimiter = ",\n"|"," | "." | "?" | "!" | "\n"
type IsTemplateTag<Word> = Word extends `{${infer TagName}}${Delimiter}${string}` ? TagName : never;
// type IsTemplateTag<Word> = Word extends `{${infer TagName}}` ? TagName : isTemplateTagWithComma<Word>;
// type isTemplateTagWithComma<Word> = Word extends `{${infer Tagname}},` ? Tagname : isTemplateTagWithPeriod<Word>;
// type isTemplateTagWithPeriod<Word> = Word extends `{${infer Tagname}}.` ? Tagname : never;

type TemplateTags<TemplateString> = TemplateString extends
  `${infer PartA} ${infer PartB}` ? IsTemplateTag<PartA> | TemplateTags<PartB>
  : IsTemplateTag<TemplateString>;


type TemplateParams<T extends string> = {
  [Key in TemplateTags<T>]: {
    toString(): string;
  };
};

type TemplateRenderFunc<T extends string> = (
  params: TemplateParams<T>,
) => string;

export function parseTemplateString<T extends string>(
  str: T,
): TemplateRenderFunc<T> {
  const tags = [...str.matchAll(/{\w+}/g)].flat();
  function render<U extends TemplateParams<T>>(params: U): string {
    // check for missing params
    if (
      tags.some((tag) => !Object.keys(params).includes(tag.slice(1, -1)))
    ) {
      throw new Error(
        `Missing template parameter. Required: ${tags.join(", ")} Received: ${
          Object.keys(params).join(", ")
        }`,
      );
    }
    let result: string = str;
    for (const key in params) {
      const value = params[key];
      result = result.replaceAll(`{${key}}`, value.toString());
    }
    return result;
  }
  return render;
}