---
title: "Removed TypeScript constructs"
description: "What Hem does not accept from JavaScript and TypeScript, and the typed replacement for each."
image: "https://hem.run/social/hem.png"
---

> Documentation Index
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# Removed TypeScript constructs

The removed constructs below are unwritable in Hem source. Removed words stay reserved as ordinary identifiers. `null` and `undefined` are admitted inside type expressions to preserve exact TypeScript contracts; their value literals remain removed.

## Removed, with replacements

| Construct | Status | Replacement |
| --- | --- | --- |
| `const` | Removed | Write `let`: Hem's `let` is TypeScript's `const` |
| `var`, `function`, `switch`, loose `==` | Removed | `let`, arrows, `match`, strict `==` |
| Truthiness, `null` and `undefined` literals | Removed | Boolean conditions, `??`, `None`, `?T` |
| `any` | Demoted on contact | `unknown` plus narrowing, cast inserted automatically |
| `enum`, `interface`, `namespace`, declaration merging | Removed | Variants, `type`, modules |
| `this` outside classes, `arguments` | Removed | Closures, rest parameters |
| Key-iterating `for..in`, C-style `for`, `do..while`, labels | Removed | Value `for..in`, ranges, `while` |
| `++`, `--`, the comma operator | Removed | `+=`, statements |
| Unchecked indexing | Removed | `xs[i]` is `?T` |
| Statement-only exception handling | Demoted | Postfix `catch` everywhere; `try` remains as a residual form |

`T | null` and `T | undefined` are exact type contracts; `?T` admits both sentinels. Use contextual `None` in value position. This type syntax does not enable unchecked assertions or authored predicate promises.

## Operator restrictions

`==` compiles to `===`. Coercing equality is not expressible. The single exception is `==` and `!=` against `None`, which emit `== null` and `!= null` so that one evaluation eliminates both nullish members.

Conditions, `!`, `&&`, and `||` accept boolean operands only. A non-boolean condition reports HEM2002.

Binary `+` and `+=` are arithmetic only: both operands `number`, or both `bigint`. Operands cannot be strings or mixed numeric types. Unary `+` requires an operand that is already `number`, rather than performing runtime conversion like `Number(x)`. All three cases report HEM2021. String concatenation has one spelling, a template literal.

## Words that are not removed

`type` and `error` are contextual keywords. They open a declaration only at the start of a statement with an identifier following, as in `type Id = string` or `error NotFound({ id: string })`.

Everywhere else they are ordinary identifiers. All of the following are plain code:

```hem
let error = validate(input)
let handler = (type: string) => type
let { type } = event
let tag = event.type
```

`brand` is contextual in one position, immediately after a type declaration's `=`. `deriving` is active only in a `deriving Eq` clause on a variant or error declaration.

## Statement results must be used

A statement whose only outcome is to compute and discard a value reports HEM2018. Two shapes are refused.

The syntactic tier covers statements that cannot have any effect: a bare name, a field or index access, a literal, or a comparison.

The type-directed tier covers a call or `await` whose checked result is a carrier by declaration origin: canonical standard `Option` or `Result`, an `Effect`, a `Promise` or `PromiseLike`, or a boxed `Option` or `Result` from the Effect transport. Canonical carriers remain must-use when Effect is disabled.

Ordinary side-effecting calls stay legal because their results are not carriers, including `array.push`, `map.set`, and `console.log`. A `?T` is not a carrier.

The deliberate discard is `let _ = expression`, which emits `void (expression);` and leaves no unused TypeScript binding.

## Emit guarantees

Emitted TypeScript contains no enums, namespaces, or parameter properties. Output runs unmodified under Node type stripping and TypeScript's `erasableSyntaxOnly`.

## Normative source

Specification 0.2, §15 (removed constructs), §6 (operators and must-use), §2 (lexical structure).

Source: https://hem.run/reference/removed-typescript/index.mdx