Correctness is held by a differential conformance suite: the same C# is evaluated in .NET and in the embedded Bun, and the two answers must agree. Anything the compiler cannot faithfully translate is a build error with a location (the EQ2xxx diagnostics), never silent wrongness.

🛡️ Boundaries (Server vs Client)

To keep browser code honest, the compiler enforces strict boundaries, inspired by Next.js (Server/Client split) and Flutter (constraints), and validates them _before_ emitting JS:

•

Client components (StatefulComponent / StatelessComponent): UI logic, state management, System.Linq, basic types (string, int, DateTime).

•

Forbidden on the client: System.IO, direct System.Net.Http, blocking .Wait(). File.ReadAllText() in a component body is a build error.

•

The bridge: data fetching goes through methods annotated with [ServerAction] (RPC style). See Security & Server Actions.

🛠️ Compiler Components

1. TypeScriptEmitter

The TypeScriptEmitter is the entry point for generating .ts files. It organizes imports, defines classes, and uses the CSharpToJsConverter to convert method bodies.

2. CSharpToJsConverter

Dispatches every Roslyn node to a strategy — one per construct (BinaryExpressionStrategy, IfStatementStrategy, …) — and returns IR, not text. Statements always build a JsStatement. An expression strategy that has crossed over builds a JsExpr (IExpressionIrStrategy); one that still returns text is spliced as an opaque node, byte-identical to what it always produced. That boundary is what lets the migration proceed one strategy at a time, and tests/eQuantic.UI.Compiler.Tests/Coverage/ir-migration.baseline.txt is the list of text strategies — it only ever shrinks, and a new strategy is born on the IR.

3. SourceMapGenerator

Generates standard V3 Source Maps with Base64 VLQ encoding, mapping generated JavaScript/TypeScript back to the original .cs or .eqx source lines.

4. Symbols first, names only where honest

The converter asks the semantic model before it guesses. A member, a local, a parameter, a static — each emits from its symbol (this.name, Widget.name, a bare name), and Console.WriteLine is console.log because the symbol says System.Console. Name heuristics (a leading underscore, a capitalised property) are legal only where the model cannot be asked — a snippet with no model, a node a strategy rewrote. Under an authoritative model an in-tree name that does not bind is a build error (EQ2006), never a guessed translation.

5. The IR and its writers

Since 0.2.0-preview.36

CodeGen/Ir/ is a small tree with one writer per level — JsExpr → JsStatement → JsClassMember → JsClass → JsModule — and the writers own everything a strategy must never hand-write:

•

Parentheses come from precedence and associativity, never from a template. Before this, f ?? g && g shipped verbatim: C# needs no parentheses there, JavaScript refuses the bare mix, and the whole bundle failed to parse.

•

Single evaluation: a JsTemplate names what it computes ({0} === {0}.normalize()) and the writer binds a part used more than once exactly once — a plain name or literal is inlined, a member read is not (a getter may count).

•

Layout: one statement per line, blocks indented (JsLayout.Pretty); Compact reproduces the former string world byte for byte, which is how each migration step is proven. A class has one layout rule — a blank line before a member with a body, fields contiguous — and a module is its imports as JsImport records, a blank line, its body.

•

The emitter (TypeScriptEmitter) decides what a module contains and hands nodes to the builder; it assembles no text.

Two nets hold all of it: the component pins (every shared component's generated module, byte for byte — a layout change must be whitespace-only against them) and the conformance suite, which executes every translated shape on both sides and compares the answers. The generated twins under src/eQuantic.UI.Runtime/src/shared/components are pinned, type-checked and tested — not linted: generated code answers to its writer, not to a style guide.

🔄 Supported Strategies

Currently, the compiler supports a wide range of C# constructs:

•

Expressions: Arithmetic, Logical, Ternary, String Interpolation, Null-coalescing (??), Conditional Access (?., ?[])

•

Control Flow: if, switch, for, foreach, while, do-while, break, continue, throw

•

Modern Patterns: Full support for Recursive, Property, Positional, Relational, and Logical patterns (C# 9.0 - 12.0)

•

Resource Management: Support for using statements and using var declarations

•

Exceptions: Full support for try-catch-finally and throw statements (Exception → Error)

•

Indexes and Ranges: Support for index-from-end operator (array[^1] → array[array.length - 1])

•

String Methods: Instance methods (Split, Replace, StartsWith, EndsWith, Contains, Substring, IndexOf, LastIndexOf, PadLeft, PadRight, Trim, TrimStart, TrimEnd, ToUpper, ToLower, ToUpperInvariant, ToLowerInvariant, Insert, Remove, ToCharArray) and static methods (IsNullOrEmpty, IsNullOrWhiteSpace, Join, Concat, Compare, Equals, Format)

•

Number Methods: int.Parse, double.Parse, float.Parse, decimal.Parse, long.Parse, int.TryParse, double.TryParse

•

List Methods: Add, AddRange, Insert, InsertRange, Remove, RemoveAt, RemoveRange, RemoveAll, Clear, IndexOf, LastIndexOf, Find, FindIndex, FindLast, FindLastIndex, FindAll, Exists, TrueForAll, Sort, ForEach, GetRange, CopyTo, BinarySearch

•

Array Static Methods: Full support for Array static methods

•

Array.Sort(array) → array.sort() - Sort array in place

•

Array.Sort(array, comparison) → array.sort(comparison) - Sort with custom comparer

•

Array.Reverse(array) → array.reverse() - Reverse array in place

•

Array.Find(array, predicate) → array.find(predicate) - Find first matching element

•

Array.FindIndex(array, predicate) → array.findIndex(predicate) - Find index of first match

•

Array.FindAll(array, predicate) → array.filter(predicate) - Find all matching elements

•

Array.IndexOf(array, value) → array.indexOf(value) - Find index of value

•

Array.LastIndexOf(array, value) → array.lastIndexOf(value) - Find last index of value

•

Array.Exists(array, predicate) → array.some(predicate) - Check if any element matches

•

Array.TrueForAll(array, predicate) → array.every(predicate) - Check if all elements match

•

Array.Clear(array) → array.splice(0) - Clear all elements

•

Array.Resize(ref array, size) → array.length = size - Resize array

•

Enum Methods: Full enum operations support

•

Enum.Parse<T>(string) → parseEnum(value, EnumType) (case-insensitive)

•

Enum.TryParse<T>(string, out var result) → (result = parseEnum(value, EnumType), result !== undefined)

•

Enum.GetValues<T>() → Object.values(EnumType) - Get all enum values

•

Enum.GetNames<T>() → Object.keys(EnumType) - Get all enum member names

•

Enum.IsDefined(typeof(T), value) → (EnumType[value] !== undefined) - Validate enum value

•

Dictionary Methods: Complete Dictionary/IDictionary support

•

ContainsKey(key) → (key in dict) - Check if key exists

•

TryGetValue(key, out var value) → (value = dict[key]) !== undefined - Safe value retrieval

•

Add(key, value) → dict[key] = value - Add or update entry

•

Remove(key) → delete dict[key] - Remove entry

•

Clear() → Object.keys(dict).forEach(k => delete dict[k]) - Remove all entries

•

Keys (property) → Object.keys(dict) - Get all keys as array

•

Values (property) → Object.values(dict) - Get all values as array

•

LINQ: Direct conversion of LINQ methods to JS equivalents:

•

Projection: Select → map, SelectMany → flatMap

•

Filtering: Where → filter, Distinct → [...new Set()]

•

Ordering: OrderBy/OrderByDescending → sort, Reverse → [...arr].reverse()

•

Partitioning: Skip → slice(n), Take → slice(0, n)

•

Element: First/FirstOrDefault → find/[0], Last/LastOrDefault → arr[arr.length-1], Single/SingleOrDefault → find/[0]

•

Quantifiers: Any → some/length > 0, All → every, Contains → includes

•

Aggregation: Count → length/filter().length, Sum → reduce((a,b) => a+b, 0), Average → reduce()/length, Min → Math.min(...), Max → Math.max(...)

•

Set Operations:

•

Concat(other) → [...source, ...other] - Concatenate two sequences

•

Union(other) → [...new Set([...source, ...other])] - Unique elements from both sequences

•

Intersect(other) → [...new Set(source)].filter(x => other.includes(x)) - Common elements

•

Except(other) → [...new Set(source)].filter(x => !other.includes(x)) - Elements in source but not in other

•

Type Filtering:

•

Cast<T>() → passthrough (JavaScript is dynamically typed)

•

OfType<T>() → filter(x => typeof x === 'type') for primitives, filter(x => x instanceof Type) for objects

•

Async/Await: Mapping of Task to Promise and native await support.

•

Modern C# Operators: Support for modern C# operators and keywords

•

Null-coalescing assignment: x ??= value → x ?? (x = value) - Assign only if null/undefined

•

nameof operator: nameof(variable) → 'variable' - Get name as string at compile time

•

default keyword: default(int) → 0, default(string) → null, default → undefined - Get default value for type

What C# gives you for free, and JavaScript does not

Two defaults are implicit in C# and absent in JavaScript. Both were emitted as nothing for a while, and both fail LATE, not at build time, and not where the cause is.

An unset value type is ZERO

Since 0.2.0-preview.22

A field of a value type is zero whether or not anyone wrote = 0. On the client it was undefined, and the two are not the same value. Reads survive by luck for as long as they are TESTS (undefined > 0 is false, which is what 0 would have said), and then the first ARITHMETIC turns it into NaN. Math.max(width, undefined) reaches the stylesheet as width:NaNpx, a rule the CSS parser drops whole: the class is computed, hashed, emitted, put on the element, and does nothing.

It shows up on client-RENDERED pages only, never on SSR or a direct load, because the server computes the same property in C# where it was 0 all along. So the two targets disagree about one field and the page that proves it is the one nobody reloads.

Every non-nullable value type now carries its default (0, false, an enum's zero member). Nullable ones do not, because there null IS the C# answer, and inventing a zero would be the same divergence pointing the other way.

A primary-constructor parameter is instance STATE

Since 0.2.0-preview.21

public sealed class TocEntry(Action<string, bool> onSeen, string id) : StatelessComponent

{

public override VisualNode Build(ComponentContext context) =>

new InView(Heading(id), visible => onSeen(id, visible)); // this.onSeen, this.id

}

Roslyn models the capture as an IParameterSymbol, so every place that asks "is this a parameter?" answers yes about something that behaves like a field. Emitted bare it compiles, the page renders, and the ReferenceError arrives whenever the callback finally fires, surfacing from inside the reconciler as a TypeError about something else entirely.

Plain models cross too

A component is not the only C# a page needs. The document model behind an editor, a small state machine, a parser: none of them are components, and all of them have to run on both targets. They transpile the same way, as their own modules, and the rules below are what makes the emission CHECKED rather than merely present.

Ranges are slices

line[start..end] → line.slice(start, end)

line[2..] → line.slice(2)

line[..^1] → line.slice(0, -1)

line[..^n] → $eq.slice(line, 0, false, n, true)

The last shape is the one JavaScript cannot say directly: ^0 means the END, while slice(0, -0) is slice(0, 0), which is empty. Anything but a positive literal after ^ therefore resolves against the length the way Index.GetOffset does. A Range stored as a VALUE is reported (EQ2004): nothing on the other side receives one, and indexing at the point of use is what it is for.

out and ref

JavaScript has neither. A method that declares them returns an OBJECT (its own value under $, each out and ref under its name) and its body moves inside a closure so every return in it keeps meaning what it meant. The call site unwraps with an arrow, which works in any expression position including inside an if:

var next = document.Replace(range, text, out var caret);

let caret: any;

let next = ($o => (caret = $o.caret, $o.$))(document.replace(range, text));

out leaves the JS parameter list (it is not passed IN); ref stays, because it is read before it is written. out _ assigns nothing.

Collections: capacity is not contents

new List<T>(x) means two opposite things depending on what x is, and only the resolved constructor can say which: new List<string>(other.Count) is an empty list sized ahead, new List<string>(other) is a copy. The first emits [], the second [...other].

Char arithmetic computes on code units

A C# char in + - * / % promotes to int and computes on the code unit, while a transpiled char is a 1-length string. When the RESULT type is numeric, char operands lower to code units (constant literals fold to the number; expressions read charCodeAt(0)), so text[i] - '0' is the digit and 'A' + col is a number, exactly as in .NET. (char)numeric lowers to String.fromCharCode. char + string stays concatenation: its result type is string, so the numeric branch never sees it.

A value from the server crosses a TYPED boundary

JavaScript has no decimal and no 64-bit integer, so the wire sends them as strings — a long as "9007199254740993", a decimal as "0.1", the date family as ISO text. The compiler knows the C# type of every state field and every Server Action's return, so it writes that knowledge into the twin as a static $hydration map, and the runtime coerces ONCE, where the value arrives, instead of every use site coercing defensively. A record names its own map, a list is [spec], a dictionary's values are { dict: spec }, and a tuple is positional ({ tuple: [...] }) because it crosses as an array. Nothing is emitted where hydration would be the identity, which is most fields.

A default is decided by the TYPE, and is never null for a value type

new int[0].SingleOrDefault() is 0 in .NET, not null, and the same goes for FirstOrDefault, LastOrDefault, ElementAtOrDefault and DefaultIfEmpty. A field declared without an initializer takes the same default: an int is 0, a bool false, a long 0n, and an ENUM its zero-valued member — which is a member-NAME string on this side, so a field left unset would otherwise render nothing at all. One table answers for both, by symbol, so a type reached through an alias (using Amount = decimal;) is not a different answer from the type itself.

A second OrderBy restarts the ordering

xs.OrderBy(a).OrderBy(b) sorts by b. The earlier ordering does not stay the primary key — it survives only as the tiebreak a stable sort gives it — so it is a different result from xs.OrderBy(a).ThenBy(b) whenever b has ties that a would break. Both cross faithfully: a chained OrderBy sorts its source first and sorts that, and ThenBy composes its key into the same comparison.

A half rounds to the EVEN neighbour, like .NET

Math.round(32.5) is 33 in JavaScript and 32 in .NET, which rounds a half to even. Everywhere the runtime mirrors a MathF.Round it uses the .NET rule, so a value computed in the browser is the value the server computed — this is what keeps a type scale's line height, and any layout derived from one, identical across hydration rather than a fraction off.

The LINQ surface is a table, and the numeric BCL is another

Most LINQ operators are one shape each — Where is filter, Aggregate is reduce with its arguments swapped — so they live as entries in a table keyed by operator and argument count, gated once, with the IR writer punctuating and binding any receiver used twice. An operator that has to REASON keeps a strategy of its own: the OrDefault family reads the element type, Sum picks a seed by it, Cast and OfType test types, Contains chooses between identity and structural equality. The same shape holds for the numeric BCL, where the modern surface (Double.AcosPi and its family) is a table of templates rather than a method each.

Where those tables have no entry, the call is a build ERROR naming the member, never a guess: a name emitted with nothing behind it is a ReferenceError at load, and a page that never renders.

Fixed-width integers settle by their type

A C# byte past 255 wraps; a JavaScript number keeps counting. The compiler reads the RESULT type of every + - * << (and of ++, -- and the compound forms) and settles the value where C# would: byte, sbyte, short, ushort and uint ALWAYS wrap (& 0xFF, << 16 >> 16, >>> 0 — packed values and hashes rely on it, so h *= 16777619 on a uint is the FNV step it is in .NET, through Math.imul). int and long wrap only where you wrote unchecked (unchecked(a * a) is 0 for 65536; a long wraps through BigInt.asIntN), because every plain i + 1 in a UI would otherwise carry a | 0 for an overflow that is a bug anywhere else — so a plain int.MaxValue + 1 keeps the double's count, a documented limit. A checked context — the block, the expression, or the project-wide setting, read from the bound tree's IsChecked rather than from the syntax — throws an overflow exactly where C# throws it. A float result is rounded to single precision (Math.fround) and prints as the shortest decimal that reads back as the same single, so 0.1f + 0.2f is "0.3" on both sides. char++ steps the character; an enum in arithmetic (day + 1, a.CompareTo(b)) computes on the value behind the member name.

Implicit conversions are settled by the bound tree

The syntax never shows an implicit conversion — int i = c with a char, Twice(c), a[c], long l = n — and a rule written against syntax covers only the shapes its author remembered. The compiler reads them from Roslyn's bound tree instead: after every expression is translated, the conversion the bound tree wraps it in is applied (ValueFlow), at every site C# applies it — initializers, arguments, indexes, returns, comparisons. A char promoted to a number becomes its code unit (a constant folds: 'A' + col is 65 + col); an int flowing into a long becomes a BigInt (1 → 1n, which is also why TimeSpan.FromSeconds(90) emits 90n: .NET 9's overload takes a long). Two chars compared stay characters — JavaScript orders 1-length strings by the same code units — and a user-defined implicit operator passes the value through, because the framework's wrappers (SizeValue, Index, ColorToken) are their primitive on this side. A value on its way into TEXT is settled the same way: boxed into a concatenation, a string operand of one, or a plain interpolation hole, it prints as C# prints it — null as nothing, a bool as "True", an enum by its member name — s += flag included, which no syntax rule had seen.

A type's own operators cross

JavaScript cannot overload +, so a record or struct you declare with operators carries each one in its twin as a static method, and every site the bound tree shows an operator at calls it: a + b is Money.opAdd(a, b), -m is Money.opNegate(m) (unary and binary - are named by arity, so they never collide), m += other is m = Money.opAdd(m, other). Conversions too: implicit operator Money(int v) becomes Money.fromInt, called wherever C# converts — a declaration, an argument, the operand of a compound — and explicit operator int(Money m) becomes Money.toInt, called by the cast. Only for types in your source: a framework wrapper such as SizeValue or Index is its primitive on this side, and its operators pass the value through.

The ELEMENT of a foreach converts the same way, one item at a time: foreach (long l in ints) makes each int a BigInt, foreach (int code in chars) each char its code unit, foreach (Money m in ints) calls the type's conversion — the loop binds a raw $l and declares l from it. A using DECLARATION (using var r = …;) owns the rest of its block: what follows runs inside a try whose finally disposes r — after a return value is taken, when the body throws, in reverse order when several share a block — and await using awaits disposeAsync.

Dictionaries enumerate as pairs

A transpiled primitive-keyed Dictionary is a plain object, and not iterable, so foreach over one (and new List<KeyValuePair<,>>(dict)) lowers through $eq.entries(obj, numericKeys): pairs that destructure as [key, value] AND answer .key/.value (both C# consumption shapes), with numeric keys restored as numbers (Object.entries strings them, and a stringified key would turn the next key + 1 into concatenation). Record/struct-keyed dictionaries keep their valueMap lowering; only primitive keys take this path.

Generic item annotations defer to inference

new List<KeyValuePair<int,float>>(…) cannot annotate its let with the bare C# name (KeyValuePair[] names nothing in TS); generic items leave the annotation to inference.

Statics initialise LAZILY

The shared library's modules import each other through one barrel, so a static field initialised at module-evaluation time can see another module's class as undefined. Any initialiser that names another module becomes a lazy getter backed by a private slot, which is also the faithful translation, since C# initialises a type's statics on first use. Pure literals stay fields.

What a type ANNOTATION may name

The emitted .ts is type-checked (that is the second of the two layers), so a signature must never introduce a name the module cannot resolve:

TypeScript

an enum

string, since its runtime representation is the member name

an interface with no emitted twin

any

IReadOnlyList<(char, char)>

[string, string][]

Action<T>?

((t: T) => void) \| null, parenthesised, or the union binds to the return

char

string

a name nothing can verify

any, because a wrong type is worse than an open one

Records carry the same rules, plus their static fields and their computed properties (a record is a value with BEHAVIOUR, not just its positional members).

The library IS its directory

Both the transpiled set and the runtime's export barrel are generated from the source directory, never from a hand-kept roster, so the embedded library can never drift from the code it is built from.

📝 Conversion Example

C# Source:

private void Increment() {

Count++;

if (Count > 10) Console.WriteLine("Max reached");

}

TypeScript Output:

increment() {

this.count++;

if (this.count > 10) console.log("Max reached");

}

🎯 Advanced Features Examples

Enum Operations

C# Source:

public enum OrderStatus { Pending, Processing, Shipped, Delivered }

private void HandleStatusChange(string input)

{

// Parse enum from string (case-insensitive)

if (Enum.TryParse<OrderStatus>(input, out var status))

{

Console.WriteLine($"Status changed to: {status}");

}

// Get all enum values for dropdown

var allStatuses = Enum.GetValues<OrderStatus>();

foreach (var s in allStatuses)

{

Console.WriteLine($"Available status: {s}");

}

// Validate enum value

if (Enum.IsDefined(typeof(OrderStatus), "Shipped"))

{

Console.WriteLine("Valid status");

}

TypeScript Output:

handleStatusChange(input: string) {

// Parse with TryParse

if ((status = parseEnum(input, OrderStatus), status !== undefined)) {

console.log(`Status changed to: ${status}`);

}

// Get all values

const allStatuses = Object.values(OrderStatus);

for (const s of allStatuses) {

console.log(`Available status: ${s}`);

}

// Validate

if ((OrderStatus['Shipped'] !== undefined)) {

console.log('Valid status');

}

Dictionary Operations

C# Source:

private Dictionary<string, int> _settings = new();

private void ManageSettings()

{

// Add entries

_settings.Add("timeout", 5000);

_settings.Add("retries", 3);

// Check existence

if (_settings.ContainsKey("timeout"))

{

var timeout = _settings["timeout"];

Console.WriteLine($"Timeout: {timeout}");

}

// Safe retrieval

if (_settings.TryGetValue("maxItems", out var max))

{

Console.WriteLine($"Max: {max}");

}

// Iterate keys

foreach (var key in _settings.Keys)

{

Console.WriteLine($"{key} = {_settings[key]}");

}

// Clear all

_settings.Clear();

}

TypeScript Output:

private _settings: Record<string, number> = {};

manageSettings() {

// Add entries

this._settings['timeout'] = 5000;

this._settings['retries'] = 3;

// Check existence

if (('timeout' in this._settings)) {

const timeout = this._settings['timeout'];

console.log(`Timeout: ${timeout}`);

}

// Safe retrieval

if ((max = this._settings['maxItems']) !== undefined) {

console.log(`Max: ${max}`);

}

// Iterate keys

for (const key of Object.keys(this._settings)) {

console.log(`${key} = ${this._settings[key]}`);

}

// Clear all

Object.keys(this._settings).forEach(k => delete this._settings[k]);

}

LINQ Set Operations

C# Source:

private void ProcessCollections()

{

var list1 = new[] { 1, 2, 3, 4 };

var list2 = new[] { 3, 4, 5, 6 };

// Concatenate two lists

var combined = list1.Concat(list2);

// Result: [1, 2, 3, 4, 3, 4, 5, 6]

// Union - unique elements from both

var union = list1.Union(list2);

// Result: [1, 2, 3, 4, 5, 6]

// Intersect - common elements

var common = list1.Intersect(list2);

// Result: [3, 4]

// Except - elements in list1 but not in list2

var difference = list1.Except(list2);

// Result: [1, 2]

// Complex filtering with set operations

var activeUsers = GetActiveUsers();

var premiumUsers = GetPremiumUsers();

// Users that are both active AND premium

var activePremium = activeUsers.Intersect(premiumUsers);

// Users that are active but NOT premium

var activeFree = activeUsers.Except(premiumUsers);

}

TypeScript Output:

processCollections() {

const list1 = [1, 2, 3, 4];

const list2 = [3, 4, 5, 6];

// Concatenate

const combined = [...list1, ...list2];

// Union (with Set to remove duplicates)

const union = [...new Set([...list1, ...list2])];

// Intersect (common elements)

const common = [...new Set(list1)].filter(x => list2.includes(x));

// Except (difference)

const difference = [...new Set(list1)].filter(x => !list2.includes(x));

// Complex filtering

const activeUsers = this.getActiveUsers();

const premiumUsers = this.getPremiumUsers();

const activePremium = [...new Set(activeUsers)].filter(x => premiumUsers.includes(x));

const activeFree = [...new Set(activeUsers)].filter(x => !premiumUsers.includes(x));

}

Array Static Methods

C# Source:

private void ProcessArrayOperations()

{

var numbers = new[] { 5, 2, 8, 1, 9 };

var items = new[] { "apple", "banana", "cherry" };

// Sort array in place

Array.Sort(numbers);

// Result: [1, 2, 5, 8, 9]

// Sort with custom comparison

Array.Sort(items, (a, b) => b.Length - a.Length);

// Result: ["banana", "cherry", "apple"]

// Reverse array

Array.Reverse(numbers);

// Result: [9, 8, 5, 2, 1]

// Find operations

var firstEven = Array.Find(numbers, n => n % 2 == 0);

var firstEvenIndex = Array.FindIndex(numbers, n => n % 2 == 0);

var allEvens = Array.FindAll(numbers, n => n % 2 == 0);

// Search operations

var index = Array.IndexOf(numbers, 5);

var lastIndex = Array.LastIndexOf(numbers, 5);

// Check operations

var hasEven = Array.Exists(numbers, n => n % 2 == 0);

var allPositive = Array.TrueForAll(numbers, n => n > 0);

// Clear and resize

Array.Clear(numbers);

Array.Resize(ref items, 5); // Expand to 5 elements

}

TypeScript Output:

processArrayOperations() {

const numbers = [5, 2, 8, 1, 9];

const items = ["apple", "banana", "cherry"];

// Sort

numbers.sort();

// Sort with comparison

items.sort((a, b) => b.length - a.length);

// Reverse

numbers.reverse();

// Find operations

const firstEven = numbers.find(n => n % 2 == 0);

const firstEvenIndex = numbers.findIndex(n => n % 2 == 0);

const allEvens = numbers.filter(n => n % 2 == 0);

// Search operations

const index = numbers.indexOf(5);

const lastIndex = numbers.lastIndexOf(5);

// Check operations

const hasEven = numbers.some(n => n % 2 == 0);

const allPositive = numbers.every(n => n > 0);

// Clear and resize

numbers.splice(0);

items.length = 5;

}

LINQ Type Filtering (Cast & OfType)

C# Source:

private void FilterByType()

{

// Mixed type collection

object[] mixed = new object[] { 1, "hello", 2, "world", 3.14, true };

// Cast<T>() - assumes all elements are of type T (passthrough in JS)

var assumedStrings = mixed.Cast<string>();

// OfType<T>() - filters to only elements of type T

var onlyStrings = mixed.OfType<string>();

// Result: ["hello", "world"]

var onlyNumbers = mixed.OfType<int>();

// Result: [1, 2]

// Works with custom classes too

var shapes = new object[] { new Circle(), new Square(), new Circle() };

var circles = shapes.OfType<Circle>();

// Result: [Circle, Circle]

// Primitive type filtering

var primitives = new object[] { 1, "text", 2.5, true, null };

var strings = primitives.OfType<string>(); // ["text"]

var numbers = primitives.OfType<double>(); // [1, 2.5]

var booleans = primitives.OfType<bool>(); // [true]

}

TypeScript Output:

filterByType() {

// Mixed type collection

const mixed = [1, "hello", 2, "world", 3.14, true];

// Cast - passthrough (JS is dynamically typed)

const assumedStrings = mixed;

// OfType - filter by typeof for primitives

const onlyStrings = mixed.filter(x => typeof x === 'string');

// Result: ["hello", "world"]

const onlyNumbers = mixed.filter(x => typeof x === 'number');

// Result: [1, 2, 3.14]

// OfType - filter by instanceof for objects

const shapes = [new Circle(), new Square(), new Circle()];

const circles = shapes.filter(x => x instanceof Circle);

// Result: [Circle, Circle]

// Primitive filtering

const primitives = [1, "text", 2.5, true, null];

const strings = primitives.filter(x => typeof x === 'string'); // ["text"]

const numbers = primitives.filter(x => typeof x === 'number'); // [1, 2.5]

const booleans = primitives.filter(x => typeof x === 'boolean'); // [true]

}

Modern C# Operators

C# Source:

private void DemonstrateModernOperators()

{

// Null-coalescing assignment (??=)

string? cachedData = null;

cachedData ??= LoadDataFromDatabase(); // Only loads if null

cachedData ??= "Default"; // Won't execute, already assigned

// Property null-coalescing assignment

if (user.Settings ??= new Settings())

{

Console.WriteLine("Created new settings");

}

// nameof operator (useful for property binding, validation)

var propertyName = nameof(user.Email);

Console.WriteLine($"Validating {propertyName}"); // "Validating Email"

var methodName = nameof(ProcessOrder);

LogAction(methodName); // "ProcessOrder"

// default keyword - type-safe default values

int count = default(int); // 0

string? text = default(string); // null

bool flag = default(bool); // false

DateTime date = default(DateTime); // 1/1/0001 12:00:00 AM

// default literal (contextual)

int number = default; // 0 (inferred from type)

ProcessData(default); // passes default value for parameter type

}

private void ProcessData(int value = default)

{

// value defaults to 0 if not provided

}

TypeScript Output:

demonstrateModernOperators() {

// Null-coalescing assignment

let cachedData = null;

cachedData ?? (cachedData = this.loadDataFromDatabase());

cachedData ?? (cachedData = 'Default');

// Property assignment

if (this.user.settings ?? (this.user.settings = new Settings())) {

console.log('Created new settings');

}

// nameof operator

const propertyName = 'Email';

console.log(`Validating ${propertyName}`);

const methodName = 'ProcessOrder';

this.logAction(methodName);

// default keyword

let count = 0;

let text = null;

let flag = false;

let date = null;

// default literal

let number = undefined;

this.processData(undefined);

}

processData(value = 0) {

// value defaults to 0

}

String Methods - Additional Examples

C# Source:

private void StringManipulation()

{

var text = " Hello World ";

// Trimming

var trimmed = text.Trim(); // "Hello World"

var leftTrim = text.TrimStart(); // "Hello World "

var rightTrim = text.TrimEnd(); // " Hello World"

// Case conversion

var upper = text.ToUpper(); // " HELLO WORLD "

var lower = text.ToLower(); // " hello world "

var upperInv = text.ToUpperInvariant(); // " HELLO WORLD "

var lowerInv = text.ToLowerInvariant(); // " hello world "

// Chaining methods

var clean = text.Trim().ToLower().Replace("world", "everyone");

// Result: "hello everyone"

}

TypeScript Output:

stringManipulation() {

const text = " Hello World ";

// Trimming

const trimmed = text.trim();

const leftTrim = text.trimStart();

const rightTrim = text.trimEnd();

// Case conversion

const upper = text.toUpperCase();

const lower = text.toLowerCase();

const upperInv = text.toUpperCase();

const lowerInv = text.toLowerCase();

// Chaining

const clean = text.trim().toLowerCase().replaceAll("world", "everyone");

}

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ON THIS PAGE

🛡️ Boundaries (Server vs Client)

🛠️ Compiler Components

1. TypeScriptEmitter

2. CSharpToJsConverter

3. SourceMapGenerator

4. Symbols first, names only where honest

5. The IR and its writers

🔄 Supported Strategies

What C# gives you for free, and JavaScript does not

An unset value type is ZERO

A primary-constructor parameter is instance STATE

Plain models cross too

Ranges are slices

out and ref

Collections: capacity is not contents

Char arithmetic computes on code units

A value from the server crosses a TYPED boundary

A default is decided by the TYPE, and is never null for a value type

A second OrderBy restarts the ordering

A half rounds to the EVEN neighbour, like .NET

The LINQ surface is a table, and the numeric BCL is another

Fixed-width integers settle by their type

Implicit conversions are settled by the bound tree

A type's own operators cross

Dictionaries enumerate as pairs

Generic item annotations defer to inference

Statics initialise LAZILY

What a type ANNOTATION may name

The library IS its directory

📝 Conversion Example

🎯 Advanced Features Examples

Enum Operations

Dictionary Operations

LINQ Set Operations

Array Static Methods

LINQ Type Filtering (Cast & OfType)

Modern C# Operators

String Methods - Additional Examples