JavaScript (Node.js) to Raku - nutshell
Learning Raku from Node.js, in a nutshell

This page attempts to provide a way for users experienced in Node.js to learn Raku. Features shared between the two languages will be explained here, as well as major differences in syntax and features.

This is not a tutorial for learning Raku; this is a reference for users who are already at an intermediate to advanced skill level with Node.js.

Basic syntax §

"Hello, world!" §

Let's start with the typical first program when learning new languages. In Node.js, a hello world program would be written like this:

console.log('Hello, world!');

Here are a couple ways to write this in the same way in Raku:

say('Hello, world!');
say 'Hello, world!';

Parentheses are optional for function calls in Raku. While semicolons are, for the most part, optional in Node.js, they are mandatory for expressions in Raku.

Now that we've greeted the world, let's greet our good friend, Joe. We'll start with Node.js again:

let name = 'Joe';
console.log('What\'s up,' + name + '?');
console.log(`What's up, ${name}?`);
console.log("What's up, ", name, "?");

Since he didn't hear us, let's greet him again, this time in Raku:

my $name = 'Joe';
say 'What\'s up, ' ~ $name ~ '?';
say "What's up, $name?";
say "What's up, ", $name, "?";

Here, there are only a couple differences: most variables in Raku have what are called sigils, which are what the $ in front of its name is, and string concatenation uses the ~ operator instead of +. What the two languages share in common here is support for string interpolation.

Now that the basic examples are out of the way, let's explain the similarities between the two languages in greater detail.

Variables §

Variables in Node.js can be defined like this;

var   foo = 1;  // Lexically scoped with functions and modules
let   foo = 1;  // Lexically scoped with blocks
const foo = 1;  // Lexically scoped with blocks; constant

// No equivalent to Raku dynamic variables exists.

global.foo = 1; // Globally scoped
foo = 1;        // Ditto, but implicit; forbidden in strict mode

In Raku there is no equivalent to var. An important note to make is that there is no variable hoisting in Raku; variables are defined and assigned at the line they're on, not defined at the top of its scope and later assigned at that line.

In addition to regular variables, in Raku there are what is known as dynamic variables. Dynamic variables are looked up using the caller's scope, rather than the outer scope. This is what the equivalent variable declarations look like in Raku:

my $foo = 1; # Lexically scoped 
our $foo = 1; # Package scoped 
my constant foo = 1; # Lexically scoped; constant 
constant foo = 1; # Package scoped; constant 
my  $*foo = 1; # Dynamic variable; lexically scoped 
our $*foo = 1; # Dynamic variable; package scoped 

GLOBAL::<$foo> := 1; # Globally scoped

Use my where you'd use let, our for variables you'd define in the outermost scope needed, and constant where you'd use const.

You may have noticed the $ and $* symbols placed before variable names. These are known as sigils and twigils respectively, and define what container the variable has. Refer to the documentation on variables for more information on sigils, twigils, and containers.

Variables in Node.js can have the same name as others from outer scopes without conflicting (though linters will usually complain about it depending on how they're configured):

let foo = 1;
function logDupe() {
    let foo = 2;
    console.log(foo);
}

logDupe(2);       // OUTPUT: 2
console.log(foo); // OUTPUT: 1

Raku also allows this:

my $foo = 1;
sub log-dupe {
    my $foo = 2;
    say $foo;
}
 
log-dupe; # OUTPUT: 2 
say $foo; # OUTPUT: 1 

Operators §

Assignment §

The = operator works the same across both languages.

The := operator in Raku binds a value to a variable. Binding a variable to another variable gives them the same value and container, meaning mutating attributes of one will mutate the other's as well. Bound variables cannot be reassigned with = or mutated with ++, --, etc. but they can be bound to another value again:

my %map;            # This is a hash, roughly equivalent to a JS object or map 
my %unbound = %map;
my %bound := %map;
%map<foo> = 'bar';
say %unbound;       # OUTPUT: {} 
say %bound;         # OUTPUT: {foo => bar} 
 
%bound := %unbound;
say %bound;         # OUTPUT: {} 

Equality §

Node.js has two equality operators: == and ===.

== is the loose equality operator. When comparing operands with the same type, it will return true if both operands are equal. However, if the operands are different types, they are both cast to their primitives before being compared, meaning these will return true:

console.log(1 == 1);   // OUTPUT: true
console.log('1' == 1); // OUTPUT: true
console.log([] == 0);  // OUTPUT: true

Similarly, in Raku, both operands are cast to Numeric before comparison if they don't share the same type:

say 1 == 1;       # OUTPUT: True 
say '1' == 1;     # OUTPUT: True 
say [1,2,3] == 3; # OUTPUT: True, since the array has three elements 

The inverse of == is !=.

Raku has another operator similar to ==: eq. Instead of casting operands to Numeric if they're different types, eq will cast them to strings:

say '1' eq '1'; # OUTPUT: True 
say 1 eq '1';   # OUTPUT: True 

The inverse of eq is ne or !eq.

=== is the strict equality operator. This returns true if both operands are the same value. When comparing objects, this will only return true if they are the exact same object:

console.log(1 === 1);   // OUTPUT: true
console.log('1' === 1); // OUTPUT: false
console.log({} === {}); // OUTPUT: false

let obj = {};
let obj2 = obj;
console.log(obj === obj2); // OUTPUT: true;

In Raku, the operator behaves the same, with one exception: two objects that have the same value, but different containers, will return false:

say 1 === 1;                    # OUTPUT: «True␤» 
say '1' === 1;                  # OUTPUT: «False␤» 
say 'ayy lmao' === 'ayy lmao';  # OUTPUT: «True␤» 
say {} === {};                  # OUTPUT: «False␤» 
 
my \hash = {};
my %hash = hash;
say hash === %hash; # OUTPUT: False 

In the last case it's the same object, but containers are different, which is why it returns False.

The inverse of === is !==.

This is where Raku's other equality operators are useful. If the values have different containers, the eqv operator can be used. This operator can be also be used to check for deep equality, which you would normally need to use a library for in Node.js:

say {a => 1} eqv {a => 1}; # OUTPUT: True 
 
my \hash = {};
my %hash := hash;
say hash eqv %hash; # OUTPUT: True 

In the case you need to check if two variables have the same container and value, use the =:= operator.

my @arr = [1,2,3];
my @arr2 := @arr;   # Bound variables keep the container of the other variable 
say @arr =:= @arr2; # OUTPUT: True 

Smartmatching §

Raku has one last operator for comparing values, but it is not exactly an equality operator. This is ~~, the smartmatch operator. This has several uses: it can be used like instanceof in Node.js, to match a regex, and to check if a value is a key in a hash, bag, set, or map:

say 'ayy lmao' ~~ Str; # OUTPUT: True 
 
my %hash = a => 1;
say 'a' ~~ %hash; # OUTPUT: True 
 
my $str = 'abc';
$str ~~ s/abc/def/; # Mutates $str, like foo.replace('abc', 'def') 
say $str;           # OUTPUT: def 

While we are talking about instanceof, the equivalent to the constructor property on Node.js objects in Raku is the WHAT attribute:

console.log('foo'.constructor); // OUTPUT: String

say 'foo'.WHAT; # OUTPUT: Str 

Numeric §

Node.js has +, -, /, *, %, and (in ES6) ** as numeric operators. When the operands are different types, similarly to the equality operators, are cast to their primitives before following through with the operation, making this possible:

console.log(1 + 2);   // OUTPUT: 3
console.log([] + {}); // OUTPUT: [object Object]
console.log({} + []); // OUTPUT: 0

In Raku, again, they are converted to a Numeric type, as before:

say 1 + 2;        # OUTPUT: 3 
say [] + {};      # OUTPUT: 0 
say {} + [1,2,3]; # OUTPUT: 3 

In addition, Raku has div and %%. div behaves like int division in C, while %% checks if one number is cleanly divisible by another or not:

say 4 div 3; # OUTPUT: 1 
say 4 %% 3;  # OUTPUT: False 
say 6 %% 3;  # OUTPUT: True 

Bitwise §

Node.js has &, |, ^, ~, <<, >>, >>>, and ~ for bitwise operators:

console.log(1 << 1);  // OUTPUT: 2
console.log(1 >> 1);  // OUTPUT: 0
console.log(1 >>> 1); // OUTPUT: 0
console.log(1 & 1);   // OUTPUT: 1
console.log(0 | 1);   // OUTPUT: 1
console.log(1 ^ 1);   // OUTPUT: 0
console.log(~1);      // OUTPUT: -2

In Raku, there is no equivalent to >>>. All bitwise operators are prefixed with +, however bitwise negation uses +^ instead of ~:

say 1 +< 1; # OUTPUT: 2 
say 1 +> 1; # OUTPUT: 0 
            # No equivalent for >>> 
say 1 +& 1; # OUTPUT: 1 
say 0 +| 1; # OUTPUT: 1 
say 1 +^ 1; # OUTPUT: 0 
say +^1;    # OUTPUT: -2 

Checking for definedness §

Javascript includes a nullish coalescing operator, ??, which progresses only if null or undefined:

undefined || null || 0 || 1 ;   // => 1
undefined ?? null ?? 0 ?? 1 ;   // => 0

This is very similar to Raku // operator:

Any || Nil || 0 || 1 ;   # => 1 
Any // Nil // 0 // 1 ;   # => 0 

Custom operators and operator overloading §

Node.js does not allow operator overloading without having to use a Makefile or build Node.js with a custom version of V8. Raku allows custom operators and operator overloading natively! Since all operators are subroutines, you can define your own like so:

multi sub infix:<||=>($a, $b) is equiv(&infix:<+=>) { $a || $b }
 
my $foo = 0;
$foo ||= 1;
say $foo; # OUTPUT: 1 

Operators can be defined as prefix, infix, or postfix. The is tighter, is equiv, and is looser traits optionally define the operator's precedence. In this case, ||= has the same precedence as +=.

Note how multi is used when declaring the operator subroutines. This allows multiple subroutines with the same name to be declared while also having different signatures. This will be explained in greater detail in the Functions section. For now, all we need to know is that it allows us to override any native operator we want:

# Using the `is default` trait here forces this subroutine to be chosen first, 
# so long as the signature of the subroutine matches. 
multi sub prefix:<++>($a) is default { $a - 1 }
 
my $foo = 1;
say ++$foo; # OUTPUT: 0 

Control flow §

if/else §

You should be familiar with how if/else looks in JavaScript:

let diceRoll = Math.ceil(Math.random() * 6) + Math.ceil(Math.random() * 6);
if (diceRoll === 2) {
    console.log('Snake eyes!');
} else if (diceRoll === 16) {
    console.log('Boxcars!');
} else {
    console.log(`Rolled ${diceRoll}.`);
}

In Raku, if/else works largely the same, with a few key differences. One, parentheses are not required. Two, else if is written as elsif. Three, the if clause may be written after a statement:

my Int $dice-roll = ceiling rand * 12 + ceiling rand * 12;
if $dice-roll == 2 {
    say 'Snake eyes!';
} elsif $dice-roll == 16 {
    say 'Boxcars!';
} else {
    say "Rolled $dice-roll.";
}

Alternatively, though less efficient, this could be written to use if after statements:

my Int $dice-roll = ceiling rand * 12 + ceiling rand * 12;
say 'Snake eyes!'        if $dice-roll == 2;
say 'Boxcars!'           if $dice-roll == 16;
say "Rolled $dice-roll." if $dice-roll != 2 && $dice-roll != 16;

Raku also has when, which is like if, but if the condition given is true, no code past the when block within the block it's in is executed:

{
    when True {
        say 'In when block!'; # OUTPUT: In when block! 
    }
    say 'This will never be output!';
}

Additionally, Raku has with, orwith, and without, which are like if, else if, and else respectively, but instead of checking whether their condition is true, they check if it's defined.

switch §

Switch statements are a way of checking for equality between a given value and a list of values and run some code if one matches. case statements define each value to compare to. default, if included, acts as a fallback for when the given value matches no cases. After matching a case, break is typically used to prevent the code from the cases that follow the one matched from being executed, though rarely this is intentionally omitted.

const ranklist = [2, 3, 4, 5, 6, 7, 8, 9, 'Jack', 'Queen', 'King', 'Ace'];
const ranks    = Array.from(Array(3), () => ranklist[Math.floor(Math.random() * ranks.length)]);
let   score    = 0;

for (let rank of ranks) {
    switch (rank) {
        case 'Jack':
        case 'Queen':
        case 'King':
            score += 10;
            break;
        case 'Ace';
            score += (score <= 11) ? 10 : 1;
            break;
        default:
            score += rank;
            break;
    }
}

In Raku, given can be used like switch statements. There is no equivalent to break since when blocks are most commonly used like case statements. One major difference between switch and given is that a value passed to a switch statement will only match cases that are exactly equal to the value; given values are smartmatched (~~) against the when values.

my     @ranklist = [2, 3, 4, 5, 6, 7, 8, 9, 'Jack', 'Queen', 'King', 'Ace'];
my     @ranks    = @ranklist.pick: 3;
my Int $score    = 0;
 
for @ranks -> $rank {
    # The when blocks implicitly return the last statement they contain. 
    $score += do given $rank {
        when 'Jack' | 'Queen' | 'King' { 10                      }
        when 'Ace'                     { $score <= 11 ?? 10 !! 1 }
        default                        { $_                      }
    };
}

If there are multiple when blocks that match the value passed to given and you wish to run more than one of them, use proceed. succeed may be used to exit both the when block it's in and the given block, preventing any following statements from being executed:

given Int {
    when Int     { say 'Int is Int';     proceed }
    when Numeric { say 'Int is Numeric'; proceed }
    when Any     { say 'Int is Any';     succeed }
    when Mu      { say 'Int is Mu'               } # Won't output 
}
 
# OUTPUT: 
# Int is Int 
# Int is Numeric 
# Int is Any 

for, while, and do/while §

There are three different types of for loops in JavaScript:

// C-style for loops
const letters = {};
for (let ord = 0x61; ord <= 0x7A; ord++) {
    let letter = String.fromCharCode(ord);
    letters[letter] = letter.toUpperCase();
}

// for..in loops (typically used on objects)
for (let letter in letters) {
    console.log(letters[letter]);
}
# OUTPUT:
# A
# B
# C
# etc.

// for..of loops (typically used on arrays, maps, and sets)
for (let letter of Object.values(letters)) {
    console.log(letter);
}
# OUTPUT:
# A
# B
# C
# etc.

Raku for loops most closely resemble for..of loops, since they work on anything as long as it's iterable. C-style loops are possible to write using loop, but this is discouraged since they're better written as for loops using ranges. Like if statements, for may follow a statement, with the current iteration being accessible using the $_ variable (known as "it"). Methods on $_ may be called without specifying the variable:

my Str %letters{Str};
%letters{$_} = .uc for 'a'..'z';
.say for %letters.values;
# OUTPUT: 
# A 
# B 
# C 
# etc. 

while loops work identically between JavaScript and Raku. Raku also has until loops, where instead of iterating until the given condition is false, they iterate until the condition is true.

do/while loops are known as repeat/while loops in Raku. Likewise with while, repeat/until loops also exist and loop until the given condition is false.

To write infinite loops in Raku, use loop rather than for or while.

In JavaScript, continue is used to skip to the next iteration in a loop, and break is used to exit a loop early:

let primes = new Set();
let i      = 2;

do {
    let isPrime = true;
    for (let prime of primes) {
        if (i % prime == 0) {
            isPrime = false;
            break;
        }
    }
    if (!isPrime) continue;
    primes.add(i);
} while (++i < 20);

console.log(primes); # OUTPUT: Set { 2, 3, 5, 7, 11, 13, 17, 19 }

In Raku, these are known as next and last respectively. There is also redo, which repeats the current iteration without evaluating the loop's condition again.

next/redo/last statements may be followed by a label defined before an outer loop to make the statement work on the loop the label refers to, rather than the loop the statement is in:

my %primes is SetHash;
my Int $i = 2;
 
OUTSIDE:
repeat {
    next OUTSIDE if $i %% $_ for %primes.keys;
    %primes{$i}++;
} while ++$i < 20;
 
say %primes; # OUTPUT: SetHash(11 13 17 19 2 3 5 7) 

do §

do is not currently a feature in JavaScript, however a proposal has been made to add it to ECMAScript. do expressions evaluate a block and return the result:

constant VERSION        = v2.0.0;
constant VERSION_NUMBER = do {
    my @digits = VERSION.Str.comb(/\d+/);
    :16(sprintf "%02x%02x%04x", |@digits)
};
say VERSION_NUMBER; # OUTPUT: 33554432 

Types §

Creating types §

In JavaScript, types are created by making a class (or a constructor in ES5 and earlier). If you've used TypeScript, you can define a type as a subset of other types like so:

type ID = string | number;

In Raku, classes, roles, subsets, and enums are considered types. Creating classes and roles will be discussed in the OOP section of this article. Creating an ID subset can be done like so:

subset ID where Str | Int;

See the documentation on subset and Junction for more information.

TypeScript enums may have numbers or strings as their values. Defining the values is optional; by default, the value of the first key is 0, the next key, 1, the next, 2, etc. For example, here is an enum that defines directions for extended ASCII arrow symbols (perhaps for a TUI game):

enum Direction (
    UP    = '↑',
    DOWN  = '↓',
    LEFT  = '←',
    RIGHT = '→'
);

Enums in Raku may have any type as their keys' values. Enum keys (and optionally, values) can be defined by writing enum, followed by the name of the enum, then the list of keys (and optionally, values), which can be done using < >, « », or ( ). ( ) must be used if you want to define values for the enum's keys. Here is the Direction enum as written in Raku:

enum Direction (
    UP    => '↑',
    DOWN  => '↓',
    LEFT  => '←',
    RIGHT => '→'
);

See the documentation on enum for more information.

Using types §

In TypeScript, you can define the type of variables. Attempting to assign a value that doesn't match the type of the variable will make the transpiler error out. This is done like so:

enum Name (Phoebe, Daniel, Joe);
let name: string = 'Phoebe';
name = Phoebe; # Causes tsc to error out

let hobbies: [string] = ['origami', 'playing instruments', 'programming'];

let todo: Map<string, boolean> = new Map([
    ['clean the bathroom', false],
    ['walk the dog', true],
    ['wash the dishes', true]
]);

let doJob: (job: string) => boolean = function (job: string): boolean {
    todo.set(job, true);
    return true;
};

In Raku, variables can be typed by placing the type between the declarator (my, our, etc.) and the variable name. Assigning a value that doesn't match the variable's type will throw either a compile-time or runtime error, depending on how the value is evaluated:

enum Name <Phoebe Daniel Joe>;
my Str $name = 'Phoebe';
$name = Phoebe; # Throws a compile-time error

# The type here defines the type of the elements of the array.
my Str @hobbies = ['origami', 'playing instruments', 'programming'];

# The type between the declarator and variable defines the type of the values
# of the hash.
# The type in the curly braces defines the type of the keys of the hash.
my Bool %todo{Str} = (
    'clean the bathroom' => False,
    'walk the dog'       => True,
    'wash the dishes'    => True
);

# The type here defines the return value of the routine.
my Bool &do-job = sub (Str $job --> Bool) {
    %todo{$job} = True;
};

Comparing JavaScript and Raku types §

Here is a table of some JavaScript types and their equivalents in Raku:

JavaScript Raku
Object Mu, Any, Hash
Array List, Array, Seq
String Str
Number Int, Num, Rat
Boolean Bool
Map Map, Hash
Set Set, SetHash

Object is both a superclass of all types in JavaScript and a way to create a hash. In Raku, Mu is a superclass of all types, though usually you want to use Any instead, which is a subclass of Mu but also a superclass of nearly every type, with Junction being an exception. When using Object as a hash, Hash is what you want to use. One key difference between Object and Hash is that Object preserves the order of its keys; Hash does not by default.

There are three types equivalent to Array. Array is most similar to Array, since it acts as a mutable array. List is similar to Array, but is immutable. Seq is used to create lazy arrays.

String and Str are for the most part used identically.

There are several different types in Raku equivalent to Number, but the three you'll most commonly see are Int, Num, and Rat. Int represents an integer. Num represents a floating-point number, making it the most similar to Number. Rat represents a fraction of two numbers, and is used when Num cannot provide precise enough values.

Boolean and Bool are for the most part used identically.

Map has both a mutable and an immutable equivalent in Raku. Map is the immutable one, and Hash is the mutable one. Don't get them mixed up! Like Map in JavaScript, Map and Hash can have any type of key or value, not just strings for keys.

Like Map, Set also has both a mutable and an immutable equivalent in Raku. Set is the immutable one, and SetHash is the mutable one.

Functions §

# TBD

Object-oriented programming §

# TBD

Asynchronous programming §

# TBD

The networking API §

Net §

In Raku, there are two APIs for dealing with networking: IO::Socket::INET (for synchronous networking), and IO::Socket::Async (for asynchronous networking).

IO::Socket::INET currently only supports TCP connections. Its API resembles that of C's socket API. If you're familiar with that, then it won't take long to understand how to use it. For example, here's an echo server that closes the connection after receiving its first message:

my IO::Socket::INET $server .= new:
    :localhost<localhost>,
    :localport<8000>,
    :listen;
 
my IO::Socket::INET $client .= new: :host<localhost>, :port<8000>;
$client.print: 'Hello, world!';
 
my IO::Socket::INET $conn = $server.accept;
my Str $msg               = $conn.recv;
say $msg; # OUTPUT: Hello, world! 
$conn.print($msg);
 
say $client.recv; # OUTPUT: Hello, world! 
$conn.close;
$client.close;
$server.close;

By default, IO::Socket::INET connections are IPv4 only. To use IPv6 instead, pass :family(PF_INET6) when constructing a server or a client.

In contrast, IO::Socket::Async supports both IPv4 and IPv6 without the need to specify which family you wish to use. It also supports UDP sockets. Here's how you would write the same echo server as above asynchronously (note that Supply.tap is multithreaded; if this is undesirable, use Supply.act instead:

my $supply = IO::Socket::Async.listen('localhost', 8000);
my $server = $supply.tap(-> $conn {
    $conn.Supply.tap(-> $data {
        say $data; # OUTPUT: Hello, world! 
        await $conn.print: $data;
        $conn.close;
    })
});
 
my $client = await IO::Socket::Async.connect('localhost', 8000);
$client.Supply.tap(-> $data {
    say $data; # OUTPUT: Hello, world! 
    $client.close;
    $server.close;
});
 
await $client.print: 'Hello, world!';

The equivalent code in Node.js looks like this:

const net = require('net');

const server = net.createServer(conn => {
    conn.setEncoding('utf8');
    conn.on('data', data => {
        console.log(data); # OUTPUT: Hello, world!
        conn.write(data);
        conn.end();
    });
}).listen(8000, 'localhost');

const client = net.createConnection(8000, 'localhost', () => {
    client.setEncoding('utf8');
    client.on('data', data => {
        console.log(data); # OUTPUT: Hello, world!
        client.end();
        server.close();
    });
    client.write("Hello, world!");
});

HTTP/HTTPS §

Raku doesn't natively support HTTP/HTTPS. However, CPAN packages such as Cro help fill the gap.

DNS §

Raku does not currently support the majority of the features that Node.js's DNS module implements. IO::Socket::INET and IO::Socket::Async can resolve hostnames, but features like resolving DNS records and reverse IP lookups are not implemented yet. There are some modules that are a work in progress, such as Net::DNS::BIND::Manage, that aim to improve DNS support.

Punycode §

Punycode support is available through the Net::LibIDN, Net::LibIDN2, and IDNA::Punycode modules on CPAN.

The filesystem API §

# TBD

Modules and packages §

# TBD

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