Feature Overview

WebAssembly launched with a deliberately minimal feature set. New capabilities are added through a standardization process that moves proposals through phases before engines ship them.

This page gives a brief overview of what each feature means. For deeper coverage, see the Extensions section of the guides.

MVP (2017)

The initial release included everything needed for a working compilation target:

• Four numeric types — i32, i64, f32, f64
• Linear memory — a resizable byte array accessed by load/store instructions
• Tables — indirect function calls via call_indirect
• Imports and exports — modules declare what they need and what they provide
• Structured control flow — block, loop, if/else, br, br_table
• A stack machine model — instructions push and pop values; no registers

This was enough for C/C++ compilers (via Emscripten) to target Wasm effectively.

Standardized extensions

These proposals have reached phase 5 (standardized) and are shipped in all major engines.

Numeric & conversion improvements

Feature	What it does
Sign extension ops	i32.extend8_s, etc. — sign-extend narrow values without masking tricks
Non-trapping float-to-int	i32.trunc_sat_f64_s, etc. — saturating conversions instead of trapping on overflow
JS BigInt ↔ i64	Pass i64 values to/from JavaScript as BigInt without manual splitting

Control flow & functions

Feature	What it does
Multi-value	Functions and blocks can return multiple values; blocks can take parameters
Tail call	return_call and return_call_indirect — guaranteed tail-call optimization
Exception handling	try_table, throw, catch — structured exceptions without unwinding hacks

Memory & data

Feature	What it does
Mutable globals	Globals can be imported/exported as mutable, not just immutable
Bulk memory	memory.copy, memory.fill, table.copy — efficient bulk operations
Extended const	Use i32.add, i32.mul, global.get in constant expressions (global initializers, etc.)
Memory64	Address memory with 64-bit indices for >4 GB heaps

Types & references

Feature	What it does
Reference types	externref and funcref — opaque host references that Wasm can store but not forge
Typed function references	ref.func, call_ref — first-class typed function references, enabling direct calls without tables
Garbage collection (GC)	struct, array, ref.cast — managed heap types for languages like Java, Kotlin, Dart

SIMD & parallelism

Feature	What it does
Fixed-width SIMD	128-bit v128 type with lane-wise operations (i32x4.add, f64x2.mul, etc.)
Threads	Shared memory, memory.atomic.* instructions, and wait/notify for synchronization

Tracking proposals

The full list of proposals and their current phase is maintained at webassembly.org/features and in the proposals repository. Notable in-progress work includes:

• Stack switching — lightweight coroutines and green threads
• Branch hinting — hint to engines which branch is more likely
• Relaxed SIMD — faster SIMD by relaxing determinism constraints
• Shared-everything threads — share more than just memory across threads
• Component model — high-level module composition and interface types