How to construct URI with URI Template

Emmanuel Gautier / September 12, 2023

2 min read

Working with URIs in software development can sometimes be a challenging task, especially when we need to construct them dynamically. This is where URI templates, defined in RFC 6570, come into play. In this article, we'll explore the concept of URI templates and take a closer look at the details outlined in the RFC.

RFC 6570

RFC 6570, titled "URI Template," is a specification that defines a compact syntax for expressing URLs and URI patterns. It provides a standardized way to generate URIs dynamically by allowing placeholders and variable substitution within a template. These templates are invaluable when building APIs, web applications, or any software that interacts with URIs.

Syntax of URI Templates

The syntax of a URI template, as defined in RFC 6570, consists of literal characters and expressions enclosed in curly braces {}. Here are some examples:

  • {scheme}://{host}/{path}

In these examples, {scheme}, {host}, {path}, {resource}, {id}, and {lang} are placeholders that can be replaced with actual values.

URI Template Expressions

RFC 6570 defines various expression types that can be used within URL templates, including:

  • Simple String Expansion: Represented as {varname} and are replaced by their corresponding values.
  • Reserved Expansion: Represented as {+varname} and are used for expansion without percent encoding. Useful for query strings.
  • Fragment Expansion: Represented as {#varname} and used for expansion within a URL fragment.
  • Label Expansion with Dot-Prefix: Represented as {.varname} and used for dot notation in path segments.
  • Path Segment Expansion: Represented as {/varname} and used for inserting values into path segments.
  • Query Expansion: Represented as {?varname} and used for adding query parameters.
  • Continuation: Represented as {&varname} and used to continue a query with additional parameters.

Implementing URI Templates in code

To implement URI templates in your code, you can use libraries or built-in functions that support RFC 6570. Many programming languages have libraries that make working with URI templates straightforward. There is an existing Wiki page listing RFC 6570 implementations.

For example, in Javascript, the std-uritemplate library provides robust support for URI templates. Here's a basic example in Javascript:

const { StdUriTemplate } = require('@std-uritemplate/std-uritemplate');

url = StdUriTemplate.expand("{resource}/{id}", { resource: "users", id: 123 })


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