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Nim for TypeScript Programmers

Juan Carlos edited this page Aug 30, 2021 · 103 revisions
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Table of contents

Comparison Variables Ternary operators Anonymous functions
This Arrow functions Compile-Time Function Execution Server-side rendering
Client-side rendering Svelte Parcel CommonJS
WebGL GUI Electron Webview React
More GUI frameworks WebAssembly Build modes Minification
Obfuscation JavaScript interoperability Pragmas for Interoperability Interoperability template
Regular expressions HTML/CSS WYSYWIG editor for Nim Run on NodeJS BigInt
Fetch Best Practices More

Unofficial, work in progress! This is still a stub. Please help to extend it. There may be inaccuracies in this guide. The guide assumes some intermediate knowledge.

The official tutorials can be found here:

The official manual provides an overview of the language:

The official library docs document Nim's standard library:


Feature TypeScript Nim
Execution model JavaScript code (Compiler) JavaScript code (Compiler)
Written using TypeScript Nim
License Apache MIT
Version (Major) 3.x 1.x
Typing Static, "provably correct" types Static, Strong, Inferred
Meta-programming :negative_squared_cross_mark: #issue-13252, Decorators are limited :heavy_check_mark: template, macro
int8/16/32/64 types :negative_squared_cross_mark: :heavy_check_mark:
float32/float64 types :negative_squared_cross_mark: :heavy_check_mark:
Char types :negative_squared_cross_mark: :heavy_check_mark:
Subrange types :negative_squared_cross_mark: #issue-225324972 :heavy_check_mark:
JSON types :negative_squared_cross_mark: #issue-56296923 :heavy_check_mark:
Regex types :negative_squared_cross_mark: #issue-128264906 :heavy_check_mark:
Option types :negative_squared_cross_mark: :heavy_check_mark:
Operator overloading :negative_squared_cross_mark: :heavy_check_mark:
Custom operators :negative_squared_cross_mark: :heavy_check_mark:
Run-time checks :negative_squared_cross_mark: :heavy_check_mark:
Side effects tracking :negative_squared_cross_mark: :heavy_check_mark:
Enum types :heavy_check_mark: :heavy_check_mark:
Immutability Limited, readonly keyword :heavy_check_mark:
Function arguments immutability Mutable Immutable
Full DOM API :heavy_check_mark: :heavy_check_mark:
console.log() anywhere :question: Compiler complains :heavy_check_mark:
console.assert() anywhere :question: Compiler complains :heavy_check_mark:
NodeJS integration :heavy_check_mark: :heavy_check_mark:
Generics :heavy_check_mark: :heavy_check_mark:
Type inference :heavy_check_mark: :heavy_check_mark:
Closures :heavy_check_mark: :heavy_check_mark:
Object-oriented :heavy_check_mark: :heavy_check_mark:
Methods :heavy_check_mark: :heavy_check_mark:
Exceptions :heavy_check_mark: :heavy_check_mark:
Anonymous functions :heavy_check_mark: :heavy_check_mark:
Arrow functions :heavy_check_mark: :heavy_check_mark:
Array comprehensions :heavy_check_mark: :heavy_check_mark:
Formatted string literals :heavy_check_mark: :heavy_check_mark:
FFI :heavy_check_mark: JS only :heavy_check_mark: C/C++/JS
Async :heavy_check_mark: :heavy_check_mark:
Regex :heavy_check_mark: :heavy_check_mark:
Self-documentation comments :heavy_check_mark: :heavy_check_mark:
Package publishing :heavy_check_mark: :heavy_check_mark:
Package manager :heavy_check_mark: :heavy_check_mark:
Code auto formatter :heavy_check_mark: via NPM :heavy_check_mark: Nimpretty
File extensions .ts, .tsx .nim, .nims


Creating a new variable uses var, let or const. Nim has immutability and compile-time function execution. You can assign functions to variables. const is different from TypeScript by being truly immutable.

Feature const let var
Run-Time NO :heavy_check_mark: YES :heavy_check_mark: YES
Compile-Time :heavy_check_mark: YES NO NO
Immutable :heavy_check_mark: YES :heavy_check_mark: YES NO
AutoInitialized :heavy_check_mark: YES :heavy_check_mark: YES :heavy_check_mark: YES
Reassignable NO NO :heavy_check_mark: YES
Requires Assignment :heavy_check_mark: YES :heavy_check_mark: YES NO
Can be Global :heavy_check_mark: YES :heavy_check_mark: YES :heavy_check_mark: YES

If you are just starting from scratch, you can use var while learning, it will not produce an error for doing so, until you learn more.

Ternary operators

conditional ? "result0" : "result1"

:arrow_up: TypeScript :arrow_up:          :arrow_down: Nim :arrow_down:

if conditional: "result0" else: "result1"

You probably notice that the Ternary Operator is just an if..else inline.

Anonymous functions

var myfunc = (argument1: number, argument2: number) => {
    return argument1 + argument2

console.log( myfunc(1, 2) )

:arrow_up: TypeScript :arrow_up:          :arrow_down: Nim :arrow_down:

var myfunc = ( proc (argument1, argument2: int): int = argument1 + argument2 )

echo myfunc(1, 2)

In Nim an anonymous function is a function without a name and surrounded by brackets.


this in Nim does not have a fixed or hard coded naming, so you may see some code using self instead. Nim only cares about it being the first argument. this or self is immutable by default.

type Cat = object

proc purr(this: Cat) = echo "Purr Purr"         # using 'this'
proc dance(self: Cat) = echo "Tappity Tappity"  # using 'self'

let garfield = Cat()

garfield.purr()   # Purr Purr  # Tappity Tappity

# yep, this/self is the instance
proc himself(self: Cat): Cat = return self
echo garfield == garfield.himself()  # true

Arrow functions

Nim has arrow functions, they are syntactic sugar for normal functions. Using arrow functions requires to import sugar.

Let's convert the above cat example to use arrow functions:

import sugar

type Cat = object

let purr = (this: Cat) => echo "Purr Purr"         # using `this`
let dance = (self: Cat) => echo "Tappity Tappity"  # using `self`

# The call syntax is identical

If you do not need to pass any arguments, don't:

import sugar

let purr = () => echo "Purr Purr"  # No arguments

purr()  # Purr Purr

You can use pragmas on your arrow functions:

let functionName = () {.inline.} => 42
let another_name = () {.inline.} => 42

(See also inline pragma)

You can use function names and pragmas on your arrow functions:

function_name(argument0, argument1: int) {.noSideEffect.} => argument0 + argument1
let variab = (argument0, argument1: int) {.noSideEffect.} => argument0 + argument1

(See also noSideEffect pragma)

Compile-time function execution

Nim has compile-time function execution that allows you to run backend-like code at compile-time and use it on frontend at run-time. Compile-time FFI is also possible. Most code that works at compile-time and NimScript also tends to work for frontend.

Server-side rendering

With SCFs

Nim Source Code Filters (SCF) are a standard library templating mechanism to do server-side rendering of templates.

SCFs have the file extension *.nimf and start with the shebang #?stdtmpl. Inside, the logic is normal Nim code, but with the prefix #, while the template is written as-is (no #).

SCFs are usually functions that return a string. Those functions can use normal string operations and formatting. They get compiled into normal Nim code, providing excellent performance.

#func generateXML(name, age: string): string =
include "serverside.nimf"

echo generateXML("Nim", "16 (in 2021)")

# prints:
# <xml>
#     <name>Nim</name>
#     <age>16 (in 2021)</age>
# </xml>

With Karax

The Karax framework also does server-side rendering (install with nimble install karax):

import karax/[karaxdsl,vdom]

writeFile "app.html", $(block: buildHtml(tdiv):
  h1: text"Hello World"
  p: text"Compile:  nim r file.nim ")
include prelude
import karax / [karaxdsl, vdom]

writeFile "example.html", $(block: buildHtml(tdiv):

  link(href="", rel="stylesheet")


      button(class="button is-success"): text"Example"

        li: text"Item 0"
        li: text"Item 1"

      code(class="code"): text"""echo "Hello World" """

Client-Side Rendering

Karax can also do client-side rendering, it includes a local dev server among other really cool things.

You can compile and run this example in web browser using Karax karun tool.

include karax/prelude
setRenderer func: auto = buildHtml(h1): text"Hello World"

Karax Clien Side Render


nclearseam is a Nim frontend inspired by Svelte.


parcel-plugin-nim is a Nim module for Parcel support.


jsExport.nim is a Nim module for CommonJS support.


nimx is a cross-platform GUI framework with WebGL target, compiles your GUI to WebGL.


You can run Nim inside Electron.


You can also use a cross-platform tiny (1 .h file) and fast (C code) webview:

WIISH also provides a similar cross-platform WebView, with other targets too.


React.nim provides ReactJS bindings for Nim.

More GUI frameworks

If it is not mandatory to be a JavaScript-only GUI, and you just need a GUI that works on Windows/Linux/Mac, then you have even more alternatives to try:


You can compile your Nim code to WebAssembly using:

More about compiling your project to WebAssembly:

Build modes

When your code is ready for production you can create a release build by adding the compile commands -d:release and -d:danger.

Feature Release Build Debug Build
Speed Fast Slow
File Size Small Big
Optimized :heavy_check_mark: :negative_squared_cross_mark:
Tracebacks :negative_squared_cross_mark: :heavy_check_mark:
Run-time checks :heavy_check_mark: :heavy_check_mark:
Compile-time checks :heavy_check_mark: :heavy_check_mark:
assert :negative_squared_cross_mark: :heavy_check_mark:
doAssert :heavy_check_mark: :heavy_check_mark:


Nim does not minify the compiled JavaScript by default.

Nim typically compiles to very small file sizes when it builds for release. Thanks to dead code elimination only the used symbols get compiled, others are not present in the release build. E.g. if you import a module but do not use it, it will not exist in the release build (and a hint in the terminal will notify you about the unused import).

Nim uses spaces as indentation. Basically you are good to go with the JavaScript file that Nim compiles.

Alternatively you can use any other minifier software to do that kind of post-processing of JavaScript.


Nim does not obfuscate the compiled JavaScript by default.

If you want to obfuscate the compiled JavaScript, you can control the name mangling, among other more useful features using {.exportc.} pragma, to use the mangling as obfuscator.

var variable {.exportc: "lkfnjmgkw3du4905r3q2ep8n4urfp34w2efltgvepotik132qm0".} = false
proc funct() {.exportc: "kl34jgo9liw35e4atr8i30q2rk1fipkpfrsdofir93o2qujfoks".} = echo 42

Compiles to:

var lkfnjmgkw3du4905r3q2ep8n4urfp34w2efltgvepotik132qm0 = false;

function kl34jgo9liw35e4atr8i30q2rk1fipkpfrsdofir93o2qujfoks() {

You use the human friendly names variable and funct, while the code compiles to the obfuscated names.

Nim will not lose track of the names, you can generate the obfuscated names with any random algo of your own.

Alternatively you can use any other obfuscator software to do that kind of post-processing of JavaScript.

JavaScript interoperability

Nim can directly interoperate with JavaScript. Here is how you can create your own libs.

There is no additional performance cost, the compiler will just emit the code you want.

As minimal as possible example, not too useful, but explains it as simple as possible:

func log(arg, arg2: SomeNumber) {.importjs: """console.log(#, #)""".}
  • # is replaced by arguments by index, starting at index 0, if any.
  • @ is replaced by all remaining arguments, if any, separated by comma.
  • $$ to escape a single $ on string or Regex.
  • $1 is replaced by the current function name.
  • Argument types and return types are Nim types, from std lib or your own custom.
  • importcpp is sometimes used as importjs.
  • You can use openArray as return type on functions, it will return array instead of seq.
  • Print to console using vanilla JavaScript console.log() and similar.
  • Ensure that DOM is ready using vanilla JavaScript (function(){ ... })();.
  • Enforce Strict Mode using vanilla JavaScript "use strict";.
  • ImportJs pragma
  • If the function has no arguments, then @ generates nothing (empty).
  • If the function has no arguments, then # produces an error.
  • Note that with @ commas are automatically inserted, with # you must add the commas.
  • You can use multi-line strings for the patterns.
  • You can use strformat for the patterns, but you must import strformat.

Let's compile this tiny example to JavaScript target:

func log(arg, arg2: SomeNumber) {.importjs: """console.log(#, #)""".}

log(42, 9)
run in console
nim js -d:danger lognumbers.nim
generates lognumbers.js
console.log(42, 9);

If you need to use named arguments you can use the {.emit.} pragma:

func functionName(namedArgument: int) = {.emit: """console.log( `namedArgument`);""".}
  • `namedArgument` is replaced by arguments by name.
  • Argument types and return types are Nim types, from std lib or your own custom.
  • Print to console using console.log() and similar.
  • You can use openArray as return type on functions, it will return array instead of seq.
  • Ensure that DOM is ready using vanilla JavaScript (function(){ ... })();.
  • You can use "use strict" if you want, but it may not have effect.
  • The rest is similar to importjs, as explained above.

Let's compile this tiny example to JavaScript target:

func log(mynum: int) = {.emit: """console.log(`mynum`);""".}

function log(mynum) {


More Examples:

func log(foo, bar, baz: SomeNumber) = {.emit:
console.log('foo = ' + `foo`, 'bar = ' + `bar`, 'baz = ' + `baz`);

func encapsulated_log(foo, bar, baz: SomeNumber | string) {.importjs:
(function () {
    "use strict";
    console.log('foo = ' + #, 'bar = ' + #, 'baz = ' + #);

log(42, 19, 0)
encapsulated_log(42, "hi", 19)

Useful pragmas for interoperability

See also:

Interoperability template

  • Nim "code template" pseudocode that you can Edit to create your own JavaScript libs:
func functionName(argument: SomeNumber | string | bool): SomeNumber {.importjs: """
(function () {
  "use strict";

  console.log("CODE HERE");
  return 42;

})(); """, exportc: "functionName".}  ## Documentation Comment.

func functionName2(namedArgument: SomeNumber | string | bool): SomeNumber = {.emit: """

  console.log("CODE HERE");
  console.log( `namedArgument` );
  return 42;

""", exportc: "functionName2".}  ## Documentation Comment.

HTML/CSS WYSYWIG editor for Nim

This is a community maintained open source project thats a HTML/CSS WYSYWIG Editor with Nim Support without JavaScript required, for creating Source Code Filter starter templates (similar to Pug templating, etc), so you dont have to start by hand with an empty blank file, not meant to produce a "finished" page but a template to edit. Ideas to make it more useful welcome. Pull Requests welcome.

  • Drag&drop your components in the page and press the "View Code" green button.
  • Documentation of each component clicking on the "?" link.
  • Ready-made full-page templates on the "Templates" category.

Spectre CSS

Uses Spectre CSS because it wont need third-party JavaScript libraries, is 9 Kilobytes, pure CSS, JavaScript framework agnostic.

Bulma CSS

Uses Bulma CSS because it wont need third-party JavaScript libraries, pure CSS, JavaScript framework agnostic.

NodeJS compatibility


To not duplicate all documentation here, please continue reading:

It is Python-oriented, but the same concepts also work for Frontend too!.

:arrow_up: :arrow_up: :arrow_up: :arrow_up: