Converting Promise Chains to Async/Await

Modern JavaScript/TypeScript codebases often need to migrate from Promise-based code to the more readable async/await syntax. Codegen provides powerful tools to automate this conversion while preserving business logic and handling complex scenarios.

You can find the complete example code in our examples repository.

Finding Promise Chains

Codegen offers multiple ways to locate Promise chains in your codebase:

• In files
• In functions
• Part of a function call chain

Promise Chains in a File

Find all Promise chains in a file:

ts_file = codebase.get_file("api_client.ts")
promise_chains = ts_file.promise_chains

print(f"Found {len(promise_chains)} Promise chains")

Promise Chains in a Function

Find Promise chains within a specific function:

ts_func = codebase.get_function("getUserData")
chains = ts_func.promise_chains

for chain in chains:
    print(f"Found chain starting with: {chain.name}")

Promise Chain starting from a Function Call

Find Promise chains starting from a specific function call:

# Assuming the function call is part of a promise chain
fetch_call = codebase.get_function("fetchUserData").function_calls[2]
chain = fetch_call.promise_chain

Converting Promise Chains

In-Place Conversion

Convert Promise chains directly in your codebase:

# Find and convert all Promise chains in a file
for chain in typescript_file.promise_chains:
    chain.convert_to_async_await()

Handle Business Logic Without In-Place Edit

Generate the transformed code without inplace edit by returning the new code as a string. This is useful when you want to add additional business logic to the overall conversion.

async_await_code = chain.convert_to_async_await(inplace_edit=False)
print("Converted code:", async_await_code)

promise_statement = chain.parent_statement
new_code = promise_statement.edit(
    f"""
    {async_await_code}

    // handle additional business logic here
    """
)

Supported Promise Chain Patterns

• Basic promise.then() statements of any length
• Catch promise.then().catch() statements of any length
• Finally promise.then().catch().finally() statements of any length
• Desctructure promise.then((var1, var2)) statements -> let [var1, var2] = await statement;
• Implicit returns -> return promise.then(() => console.log("hello"))
• Top level variable assignments -> let assigned_var = promise.then()
• Top level variable assignments -> let assigned_var = promise.then()
• Ambiguous/conditional return blocks

A list of all the covered cases can be found in the example notebook.

Examples

1. Basic Promise Chains

// Before
function getValue(): Promise<number> {
    return Promise.resolve(10)
        .then(value => value * 2);
}

Applying the conversion…

promise_chain = codebase.get_function("getValue").promise_chains[0]
promise_chain.convert_to_async_await()
codebase.commit()

// After
async function getValue(): Promise<number> {
    let value = await Promise.resolve(10);
    return value * 2;
}

2. Error Handling with Catch/Finally

// Before
function processData(): Promise<void> {
    return fetchData()
        .then(data => processData(data))
        .catch(error => {
            console.error("Error:", error);
            throw error;
        })
        .finally(() => {
            cleanup();
        });
}

Applying the conversion…

promise_chain = codebase.get_function("processData").promise_chains[0]
promise_chain.convert_to_async_await()
codebase.commit()

// After
async function processData(): Promise<void> {
    try {
        let data = await fetchData();
        return processData(data);
    } catch (error) {
        console.error("Error:", error);
        throw error;
    } finally {
        cleanup();
    }
}

3. Promise.all with Destructuring

// Before
function getAllUserInfo(userId: number) {
    return Promise.all([
        fetchUserData(userId),
        fetchUserPosts(userId)
    ]).then(([user, posts]) => {
        return { user, posts };
    });
}

Applying the conversion…

promise_chain = codebase.get_function("getAllUserInfo").promise_chains[0]
promise_chain.convert_to_async_await()
codebase.commit()

// After
async function getAllUserInfo(userId: number) {
    const [user, posts] = await Promise.all([
        fetchUserData(userId),
        fetchUserPosts(userId)
    ]);
    return { user, posts };
}

4. Handling Ambiguous Returns Using Anonymous functions

For then blocks that have more than one return statement, Codegen will add an anonymous function to handle the ambiguous return to guarantee a deterministic conversion.

// Before
function create(opts: any): Promise<any> {
	let qResponse = request(opts);
	qResponse = qResponse.then(function success(response) {
		if (response.statusCode < 200 || response.statusCode >= 300) {
			throw new Error(JSON.stringify(response));
		}
		if (typeof response.body === "string") {
			return JSON.parse(response.body);
		}
		return response.body;
	});

	return qResponse;
}

Applying the conversion…

promise_chain = codebase.get_function("create").promise_chains[0]
promise_chain.convert_to_async_await()
codebase.commit()

// After
async function create(opts): Promise<any> {
	let qResponse = request(opts);
	let response = await qResponse;
	qResponse = (async (response) => {
		if (response.statusCode < 200 || response.statusCode >= 300) {
			throw new Error(JSON.stringify(response));
		}
		if (typeof response.body === "string") {
			return JSON.parse(response.body);
		}
		return response.body;
	})(response);

	return qResponse;
}

Handling Top-Level Assignment Variables

When converting Promise chains that involve top-level assignment variables, you can specify the variable name of your choice or pick the default which is the original variable assignment name.

# Convert with custom variable names for clarity
chain.convert_to_async_await(
    assignment_variable_name="operationResult",
)

Next Steps

Converting Promise chains to async/await improves code readability and maintainability. Codegen’s tools make this migration process automated and reliable, handling complex cases while preserving business logic. Here are some next steps to ensure a successful migration:

1. Ensure to run npx prettier --write . after the migration to fix indentation + linting
2. Incremental Migration: Convert one module at a time
3. Handle Additional Business Logic: Use .promise_statement.edit() to modify the entire chain and handle external business logic
4. If the specific conversion case is not covered, open an issue on the Codegen repository or try to right your own transformation logic using the codegen-sdk