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7.1.0 • Public • Published

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A secure, fast and lightweight template engine with some handy additions.

Think of it as a sweet spot between plain text interpolation and mustache.js; Certainly not as logic-ful as Handlebars! Sometimes a stricter syntax is the right boundary to reduce potential errors and improve performance.

  • 🏃 2x-3x faster than MustacheJS (Micromustache is the fastest template engine that doesn't need pre-compilation and still works in CSP environments)
  • 🔒 Secure has limits for variable length, number of interpolations, nesting debth and common Javascript pitfalls (__proto__, constructor, getters/etc). Works in CSP environments (no usage of eval() or new Function()). Published only with 2FA. No regexp.
  • 🎈 Lightweight No dependencies, less than 400 lines of source code, easy to audit, small API surface, easy to pick up
  • 🐁 Small memory footprint sane caching strategy, no memory leak
  • 🏳 No dependencies
  • Bracket notation support a[1]['foo'] accessors (mustache.js/handlebar syntax of a.1.foo is also supported).
  • 🚩 Meaningful errors in case of template syntax errors to make it easy to spot and fix. All functions test their input contracts and throw meaningful errors to improve developer experience (DX).
  • TypeScript types included out of the box and updated with every version of the library
  • 🐇 Works in node (CommonJS) and Browser (UMD) and EcmaScript 6 Modules (ESM)
  • 🛠 Well tested (full test coverage over 120+ tests). Also tested to produce the same results as Mustache.js.
  • 📖 Full JSDoc documentation

If variable interpolation is all you need, micromustache is a drop-in replacement for MustacheJS (see its differences with Mustache.js)

Try it in your browser!

Getting started

Use directly with UNPKG:

import { render } from 'https://unpkg.com/browse/micromustache/dist/esm.js'
console.log(render('Hello {{name}}!', { name: 'world' }))
// Hello world!


$ npm i micromustache


const { render } = require('micromustache')
console.log(render('Hello {{name}}!', { name: 'world' }))
// Hello world!

Why not just use EcmaScript template literals?

Template literals work great when the template and the variables are in the same scope but not so well when the template is in another scope or is not known ahead of time. For example, suppose you had a function like this:

function greet(name) {
  return `Hi ${name}!`

After your function became successful and you got rich 🤑 you may decide to dominate the world and expand to new markets which speak other languages. You need to internationalize it. Adding one more language is easy:

function greet(name, lang) {
  // Note the lang parameter that contains a language code
  return lang === 'sv' ? `Hej ${name}!` : `Hi ${name}!`

But how about a bunch of them?

function greet(name, lang) {
  switch (lang) {
    case 'sv': return `Hej ${name}!`
    case 'es': return `Hola ${name}!`
    case 'en': return `Hi ${name}!`

That doesn't scale well as you dominate country after country and need to support more languages! Besides, that's just one string! The main problem is that the content (the text) is coupled to the code (the variable interpolation). Template engines help you to move the content out of the function and let something else deal with that concern.

const { render } = require('micromustache')
// A very simplified i18n database
const db = {
  en: {
    greeting: 'Hi {{name}}!',
    // ...
  sv: {
    greeting: 'Hej {{name}}!',
    // ...
  // ...
function greet(name, lang) {
  return render(db[lang].greeting, { name } )

Now it's better! 😎 All the templates are together and they are easy to update and translate. By default, we use the popular syntax that encloses variable names between double curly braces ({{ and }}) but you can customize micromustache if you prefer something else. Just like template literals, you can of course reference deep nested objects:

const { render } = require('micromustache')
const scope = {
  fruits: [
    { name: 'Apple', color: 'red' },
    { name: 'Banana', color: 'yellow' },
console.log(render('I like {{fruits[1].color}}!', scope))
// I like Bababa!

It worth to note that Mustache and Handlebars don't support fruits[1].color syntax and rather expect you to write it as fruits.1.color.

The real power of micromustache comes from letting you resolve a variable name using your own functions! To pass a resolver function, you can use renderFn() instead of render():

const { renderFn } = require('micromustache')
// Just converts the variable name to upper case
const up = str => str.toUpperCase()
console.log(renderFn('My name is {{Alex}}!', up))
// My name is ALEX!

The resolver gets the scope as its second parameter. If you want to lookup a value, there's a get() function as well:

const { renderFn, get } = require('micromustache')
// Looks up the value and converts it to stars
function star(varName, scope) {
  // varName comes from the template and is 'password' here
  // scope is { password: 'abc' }
  const value = get(scope, varName) // value is 'abc'
  return '*'.repeat(value.length)
console.log(renderFn('My password is {{password}}!', star, { password: 'abc' }))
// My password is ***!

If you want to resolve a value asynchronously, we got you covered using the renderFnAsync() instead of renderFn(). For example the following code uses node-fetch to resolve a url.

const { renderFnAsync } = require('micromustache')
const fetch = require('node-fetch')
async function taskTitleFromUrl(url) {
  const response = await fetch(url)
  const obj = await response.json()
  return obj.title
console.log(await renderFnAsync('Got {{https://jsonplaceholder.typicode.com/todos/1}}!', fetch))
// Got delectus aut autem!

If you find yourself working on a particular template too often, you can compile() it once and cache the result so the future renders will be much faster. The compiler returns an object with render(), renderFn() and renderFnAsync() methods. The only difference is that they don't get the template and only need a scope:

const { compile } = require('micromustache')
const compiled = compile('Hello {{name}}! I am {{age}} years old!')
console.log(compiled.render({ name: 'world', age: 42 }))
// Hello world! I'm 42
// The methods are bound so you can use the destructed version for brevity
const { render } = compile
console.log(render({ name: 'world', age: 42 }))
// Hello world! I'm 42

If the compiled variable above is garbage collected, the cache is freed (unlike some other template engines that dearly keep hold of the compiled result in their cache which may leads to memory leaks or out of memory errors over longer usage).

Using the options you can do all sorts of fancy stuff. For example, here is an imitation of the C# string interpolation syntax:

const { render } = require('micromustache')
const $ = scope => strings => render(strings[0], scope, { tags: ['{', '}'] })
const name = 'Michael'
console.log($({ name })`Hello {name}!`)
// Hello Michael!


render(template, scope, options)

Replaces every {{varName}} inside the template with values from the scope parameter.

  • template The template string containing zero or more {{varName}} as placeholders for looking up values from the scope parameter.
  • scope?: object An object containing values for variable names from the the template. If it's omitted, we default to an empty object. Since functions are objects in javascript, the scope can technically be a function too but it won't be called. It'll be treated as an object and its properties will be used for the lookup.
  • options?: object see below 👇

The template string where its variable names replaced with corresponding values

  • TypeError if the template is not a string, scope is not an object, or options is invalid
  • SyntaxError if the template does not comply with the syntax

renderFn(template, resolveFn, scope, options)

Same as render but accepts a function that allows you to resolve the variable name to a value as you choose. Tip: you may do some extra processing and use the get() function underneath but that's up to you.

WARNING: When dealing with user input, always make sure to validate it.

New params
  • resolveFn: (varName, scope) => any a function that takes a variable name and resolves it to a value. The value can be a number, string or boolean. If it is not, it'll be "stringified".

WARNING: When dealing with user input, always make sure to validate it.

renderFnAsync(template, resolveFnAsync, scope, options)

Same as renderFn() but expects a resolver function that always returns a promise.

New params
  • resolveFnAsync: (varName, scope) => Promise<any> a function that takes a variable name and returns a promise that resolves to a value. The value can be a number, string or boolean. If it is not, it'll be "stringified".

A promise that resolves to the final output once all the resolveFnAsync functions are resolved

compile(template, options)

Compiles a template and returns an object that has the render functions. This drammatically improves the interpolation speed (2x-3x) compared to render().

  • template same template that is passed to render()
  • options?: object see below 👇

An object with 3 methods:

  • render(scope) same as the render() function above but without the template parameter
  • renderFn(resolveFn, scope) same as the renderFn() function above but without the template parameter
  • renderFnAsync(resolveFnAsync, scope) same as the renderFnAsync() function above but without the template parameter
  • TypeError if the template is not a string or options is invalid.
  • SyntaxError if the template does not comply with the syntax.

get(scope, varName, propExists)

A useful utility function that is used internally to lookup a variable name as a path to a property in an object.

  • scope same as the scope parameter to the render() function above.
  • varName: string | string[] a string like a.b.c or a['b'].c. It can also be a path array like ['a', 'b', 'c'] (same as lodash's get()). Tip: the array version is a whole lot faster because we skips parsing it.
  • propExists?: boolean = false see the meaning of this param under the options below.

Differences with JavaScript:

  • No support for keys that include [ or ]. ex. a['[']
  • No support for keys that include ' or ". ex. a['"']
  • foo[bar] is allowed and treated as foo['bar'] (this behaviour is similar to how lodash get() works). But JavaScript treats bar as a variable and tries to lookup its value or throws a ReferenceError if there is no variable called bar.

The value or undefined. If the scope is undefined or null the result is always undefined.

  • ReferenceError if it cannot find a value in the specified path and propExists is set.


All the functions that can take an option, expect it as an object with these properties:

  • explicit?: boolean = false When set to a truthy value, rendering literally puts a 'null' or 'undefined' for values that are null or undefined. By default it swallows those values to be compatible with Mustache.
  • propsExist?: boolean = false When set to a truthy value, we throw a ReferenceError for invalid varNames. Invalid varNames are the ones that do not exist in the scope. By default, invalid varNames will be resolved to an empty string. If a value does not exist in the scope, two things can happen:
    • if propsExist is falsy (default), the value will be resolved to an empty string
    • if propsExist is truthy, a ReferenceError will be thrown
  • validateVarNames?: boolean = false When set to a truthy value, validates the variable names
  • tags?: string[2] = ['{{', '}}'] The string symbols that mark the opening and closing of a variable name in the template.

Full API docs (beta)


On wiki

Known issues

On wiki

Made in Sweden 🇸🇪 by @alexewerlof


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