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import { createDeferredValue } from './deferred-value.ts';
import type { Timeout } from './types.ts';
/**
* Configuration options for the debounce utility.
*/
export type DebounceOptions<TArgs extends unknown[] = unknown[]> = {
/**
* The delay in milliseconds to wait before executing the debounced function.
*/
readonly delay: number;
/**
* If true, the function will be called immediately, i.e., on the leading edge of the timeout. If false (default), the
* function will be called on the trailing edge.
*/
readonly leading?: boolean;
/**
* If true, the debounce will wait for the previous promise to complete before processing new calls. If false
* (default), new calls will be debounced immediately regardless of previous promise state.
*/
readonly waitForPrevious?: boolean;
/**
* Optional function to accumulate arguments from multiple calls before execution. If provided, instead of using only
* the last call's arguments, this function will be called to combine the previous accumulated arguments with the new
* call's arguments.
*
* **Important**: The accumulator function is called immediately on every debounced function call (it is NOT
* debounced). Therefore, it should be fast and free of side effects to avoid performance issues.
*
* @param previousArgs - The previously accumulated arguments (undefined for the first call)
* @param currentArgs - The arguments from the current call
* @returns The accumulated arguments to use for the next call or final execution
*/
readonly accumulator?: (previousArgs: TArgs | undefined, currentArgs: TArgs) => [...TArgs];
};
/**
* A debounced function that returns a promise.
*
* @template TArgs - The arguments type of the original function.
* @template TReturn - The return type of the original function.
*/
export type DebouncedFunction<TArgs extends unknown[], TReturn> = {
/**
* Calls the debounced function and returns a promise that resolves to the result. If the function is called multiple
* times within the debounce delay, only the last call will be executed, and all callers will receive the same
* result.
*/
(...args: TArgs): Promise<TReturn>;
/**
* Cancels any pending debounced function calls.
*/
cancel(): void;
/**
* Immediately executes the debounced function with the last provided arguments, bypassing the delay.
*/
flush(): Promise<TReturn> | undefined;
};
/**
* Creates a debounced version of a function that supports promises.
*
* When the debounced function is called multiple times within the delay period, only the last call will be executed.
* All callers will receive the same result through their returned promises.
*
* By default, if the original function returns a promise, new calls will be debounced immediately without waiting. Set
* `waitForPrevious: true` to wait for previous promises to complete.
*
* @example Basic usage
*
* ```ts
* const search = debounce(async (query: string) => {
* const response = await fetch(`/api/search?q=${query}`);
* return response.json();
* }, 300);
*
* // These calls will be debounced - only the last one will execute
* const result1 = search('hello');
* const result2 = search('hello world');
* const result3 = search('hello world!');
*
* // All three promises will resolve to the same result
* const [r1, r2, r3] = await Promise.all([result1, result2, result3]);
* console.log(r1 === r2 && r2 === r3); // true
* ```
*
* @example With leading edge execution
*
* ```ts
* const saveData = debounce(
* async (data: any) => {
* return await api.save(data);
* },
* { delay: 1000, leading: true },
* );
*
* // First call executes immediately, subsequent calls are debounced
* await saveData({ id: 1 }); // Executes immediately
* await saveData({ id: 2 }); // Debounced
* ```
*
* @example With argument accumulation
*
* ```ts
* const batchProcessor = debounce(
* async (ids: number[]) => {
* return await api.processBatch(ids);
* },
* {
* delay: 300,
* // Accumulator is fast and pure - just combines arrays
* accumulator: (prev, current) => {
* const prevIds = prev?.[0] || [];
* const currentIds = current[0];
* return [[...prevIds, ...currentIds]];
* },
* },
* );
*
* batchProcessor([1, 2]);
* batchProcessor([3, 4]);
* batchProcessor([5]);
* // Eventually processes [1, 2, 3, 4, 5] in a single call
* ```
*
* @example With waiting for previous promises
*
* ```ts
* const sequentialDebounce = debounce(
* async (value: string) => {
* await longRunningOperation(value);
* return `processed: ${value}`;
* },
* { delay: 300, waitForPrevious: true },
* );
*
* sequentialDebounce('first'); // Starts long operation
* sequentialDebounce('second'); // Waits for first to complete before debouncing
* ```
*
* @template TArgs - The arguments type of the original function.
* @template TReturn - The return type of the original function.
* @param fn - The function to debounce.
* @param options - The debounce delay in millisecond or the configuration options.
* @returns A debounced version of the function that returns a promise.
*/
export const debounce = <TArgs extends unknown[], TReturn>(
fn: (...args: TArgs) => TReturn | Promise<TReturn>,
options: number | DebounceOptions<TArgs>,
): DebouncedFunction<TArgs, TReturn> => {
let timeoutId: Timeout | undefined;
let lastArgs: TArgs | undefined;
let pendingDeferred: DeferredValue<TReturn> | undefined;
let isExecuting = false;
let hasLeadingBeenCalled = false;
options =
typeof options === 'number'
? { delay: Math.max(0, Math.ceil(options)), waitForPrevious: false }
: { waitForPrevious: false, ...options, delay: Math.max(0, Math.ceil(options.delay)) };
const executeFunction = async (args: TArgs): Promise<TReturn> => {
Iif (isExecuting && options.waitForPrevious) {
// If we're already executing a promise and waitForPrevious is true,
// wait for it to complete and return the result to all pending callers
return pendingDeferred!.promise;
}
isExecuting = true;
try {
const result = await fn(...args);
Eif (pendingDeferred) {
pendingDeferred.resolve(result);
}
return result;
} catch (error) {
Eif (pendingDeferred) {
pendingDeferred.reject(error);
}
throw error;
} finally {
isExecuting = false;
pendingDeferred = undefined;
lastArgs = undefined;
}
};
const debouncedFunction = (...args: TArgs): Promise<TReturn> => {
// Handle argument accumulation
lastArgs = options.accumulator ? options.accumulator(lastArgs, args) : args;
// If we already have a pending deferred, reuse it
if (pendingDeferred) {
return pendingDeferred.promise;
}
// Create a new deferred for this execution cycle
pendingDeferred = createDeferredValue<TReturn>();
// Handle leading edge execution
if (options.leading && !hasLeadingBeenCalled && !isExecuting) {
hasLeadingBeenCalled = true;
// Execute immediately but still set up the timeout for trailing edge reset
const promise = executeFunction(args);
// Set up timeout to reset leading edge flag
Iif (timeoutId !== undefined) {
clearTimeout(timeoutId);
}
timeoutId = setTimeout(() => {
hasLeadingBeenCalled = false;
timeoutId = undefined;
}, options.delay);
return promise;
}
// Clear any existing timeout
Iif (timeoutId !== undefined) {
clearTimeout(timeoutId);
}
const debouncedExecution = async () => {
timeoutId = undefined;
hasLeadingBeenCalled = false;
Eif (lastArgs && !isExecuting) {
try {
await executeFunction(lastArgs);
} catch {
// Error is already handled by executeFunction through pendingDeferred.reject
// No need to re-throw here as it would create an unhandled rejection
}
}
};
timeoutId = setTimeout(() => void debouncedExecution(), options.delay);
return pendingDeferred.promise;
};
debouncedFunction.cancel = () => {
if (timeoutId !== undefined) {
clearTimeout(timeoutId);
timeoutId = undefined;
}
hasLeadingBeenCalled = false;
lastArgs = undefined;
if (pendingDeferred && !pendingDeferred.settled) {
pendingDeferred.reject(new Error('Debounced function call was cancelled'));
pendingDeferred = undefined;
}
};
debouncedFunction.flush = (): Promise<TReturn> | undefined => {
if (timeoutId !== undefined) {
clearTimeout(timeoutId);
timeoutId = undefined;
}
if (lastArgs && !isExecuting) {
hasLeadingBeenCalled = false;
return executeFunction(lastArgs);
}
return undefined;
};
return debouncedFunction;
};
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