String Template Tester: Live Variable Substitution for Mustache, Handlebars, and More
String templates separate static text from dynamic data — you write a pattern once and substitute variables at runtime. They power email notifications, code generation, configuration files, and dynamic web content. Different template engines use different delimiter syntax and support different features: Mustache is deliberately logic-less, Handlebars adds conditionals and loops, Jinja2 adds filters, and ES6 template literals are native JavaScript. This tool lets you test substitution live before integrating templates into your codebase.
Template engines differ in delimiter syntax, escaping rules, and control flow support. Mustache is logic-less; Handlebars adds conditionals and loops.
Comparing Template Syntax Across Engines
Mustache/Handlebars: double curly braces {{variable}}; triple braces {{{raw}}} for unescaped HTML. Jinja2 (Python/Django): {{ variable }} for output, {% if %}...{% endif %} for control flow, {{ value | filter }} for transformations. ES6 template literals: backtick syntax with ${expression} — full JavaScript expressions allowed. Go text/template: {{.Field}} with dot notation for struct fields.
Escaping and Template Injection Risks
Never use untrusted user input as the template itself — only as data substituted into a template you control. Template injection (using user input as the template string) allows attackers to execute arbitrary logic in Jinja2 and Handlebars. All major template engines HTML-escape output by default; only bypass escaping (triple braces, | safe filter) when you trust the source. SQL templates are especially dangerous — use parameterized queries, not string substitution.
Common Template Use Cases
Email templates: subject lines, greeting personalization, order confirmations. Code generation: scaffold files, API clients, database migrations. Configuration files: Kubernetes manifests with environment-specific values via Helm templates. Notification messages: Slack/Discord bots with dynamic context. Static site generation: Hugo, Jekyll, and Eleventy all use template engines to build HTML from content data.
Template Syntax Comparison
- Mustache: {{variable}} — logic-less, HTML-escaped by default
- Handlebars: {{variable}}, {{#if}}, {{#each}} — logic with helpers
- Jinja2: {{ variable }}, {% if %}, {{ val | filter }} — Python
- ES6 template literals: `${expression}` — any JS expression
- Go text/template: {{.Field}}, {{range .Items}} — struct-based
Safe vs Unsafe Template Usage
- Safe: user data as substitution values only
- Safe: HTML-escaping enabled (default in most engines)
- Unsafe: user input used as the template string itself (injection risk)
- Unsafe: triple braces / raw output with user-provided HTML
- Unsafe: string concatenation instead of parameterized queries for SQL
Frequently Asked Questions
What is a string template?
A string template is a text pattern containing placeholders (variables) replaced with real values at runtime. Templates separate the static structure of text from the dynamic data injected into it, making content reusable and data-driven without duplicating the structure each time.
What is the difference between Mustache and Handlebars?
Mustache is a logic-less template language — it supports variable substitution, sections (conditionals/loops based on truthiness), and partials. Handlebars is a superset of Mustache that adds built-in helpers (if, unless, each, with) and custom helper registration. Handlebars templates are valid Mustache templates.
What is template injection and how do I prevent it?
Template injection occurs when user-supplied input is used as the template string itself (not just as data values). An attacker can inject template syntax to execute arbitrary logic. Prevention: always use user input as data values only, never as the template pattern itself.
Can I use templates for SQL queries?
No — never use string templates for SQL. Use parameterized queries (prepared statements) instead. String-substituting user input into SQL creates SQL injection vulnerabilities. Parameterized queries send values separately from the query structure, and the database handles escaping correctly.
What is an ES6 template literal?
ES6 template literals are JavaScript's native string template syntax using backticks instead of quotes. Expressions inside ${...} are evaluated and interpolated: const greeting = `Hello, ${name}!`. Unlike Mustache/Handlebars, template literals support any JavaScript expression — arithmetic, function calls, ternary operators.
What is Jinja2?
Jinja2 is a popular Python template engine used in Flask, Django (as its template language basis), and Ansible. It uses {{ variable }} for output, {% tag %} for control flow (if, for, block, extends), and {# comment #} for comments. It also supports filters ({{ value | upper }}) and macros.
How do I iterate over a list in a Handlebars template?
Use the {{#each items}} helper: {{#each items}}