AssemblyScript for The Graph

The graph/ directory uses AssemblyScript (not TypeScript) for The Graph subgraphs. AssemblyScript compiles to WebAssembly and has critical differences from TypeScript.

Critical Constraints

No Closures

Array methods cannot capture outer variables:

typescript

// WRONG - closure captures `targetId`
const index = items.findIndex((item) => item.id == targetId);

// CORRECT - use manual iteration
let index = -1;
for (let i = 0; i < items.length; i++) {
  if (items[i].id == targetId) {
    index = i;
    break;
  }
}

BigInt Comparison

Use methods, not operators:

typescript

import { ONE, ZERO } from "../common/constants";

// WRONG
if (amount > 0) { ... }
if (amount === ZERO) { ... }

// CORRECT
if (amount.gt(ZERO)) { ... }
if (amount.equals(ZERO)) { ... }
if (amount.lt(ONE)) { ... }

String Comparison

Use == or the helper, never ===:

typescript

import { areStringsEqual } from "../common/strings";

// WRONG - strict equality compares object references
if (name === "linear") { ... }

// CORRECT
if (name == "linear") { ... }
if (areStringsEqual(name, "linear")) { ... }

No Spread Operators

typescript

// WRONG
const newObj = { ...oldObj, newField: value };

// CORRECT
const newObj = new MyType();
newObj.field1 = oldObj.field1;
newObj.field2 = oldObj.field2;
newObj.newField = value;

No Nullish Coalescing or Optional Chaining

typescript

// WRONG
const value = obj?.field ?? defaultValue;

// CORRECT
const value = obj !== null ? obj.field : defaultValue;

Type Assertions

Use changetype<T>(), not as T:

typescript

// WRONG
const addr = value as Address;

// CORRECT
const addr = changetype<Address>(value);

Dependencies

Only @graphprotocol/graph-ts is allowed. No other npm packages.

Common Patterns

Error Handling with try_*()

Contract calls can revert. Use try_ methods:

typescript

const contract = ERC20.bind(address);
const decimals = contract.try_decimals();

if (decimals.reverted) {
  return ZERO;
}
return BigInt.fromI32(decimals.value);

Entity Load/Save

typescript

let asset = Entity.Asset.load(id);
if (asset === null) {
  asset = new Entity.Asset(id);
  asset.address = address;
  asset.chainId = chainId;
}
asset.symbol = symbol;
asset.save();

Array Iteration

typescript

// Process all items
for (let i = 0; i < segments.length; i++) {
  const segment = segments[i];
  // Process segment
}

// Find with condition
let found: Segment | null = null;
for (let i = 0; i < segments.length; i++) {
  if (segments[i].amount.gt(ZERO)) {
    found = segments[i];
    break;
  }
}

Function Pointers (Generic Array Operations)

typescript

function convertItems<T>(items: T[], getValue: (item: T) => BigInt): BigInt[] {
  const result: BigInt[] = [];
  for (let i = 0; i < items.length; i++) {
    result.push(getValue(items[i]));
  }
  return result;
}

Project Utilities

Import with relative paths from your subgraph directory:

Utility	Import	Purpose
`ONE`, `ZERO`	`../common/constants`	BigInt constants
`areStringsEqual()`	`../common/strings`	Safe string comparison
`logDebug/Error/Info/Warning`	`../common/logger`	Prefixed logging
`getDay()`	`../common/helpers`	Timestamp to day
`Id` namespace	`../common/id`	Entity ID generation

Id Namespace Usage

typescript

import { Id } from "../common/id";

const streamId = Id.stream(contractAddress, tokenId);
const assetId = Id.asset(chainId, assetAddress);
const actionId = Id.action(event, contractAddress, chainId);