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after()

after<Args, Return>(func, n): (...args) => Return | undefined

Creates a function that invokes func after it's called n or more times.

Type Parameters​

Args: Args extends unknown[]​

The argument types of the function.

Return: Return​

The return type of the function.

Parameters​

func: (...args) => Return​

The function to restrict.

n: number​

The number of calls before func is invoked.

Returns​

The new restricted function. Returns undefined for the first n-1 calls.

Throws​

RangeError If n is negative or not an integer.

See Also​

before - Inverse operation: invokes func at most n-1 times.

Since​

2.0.0

Also known as​

after (Lodash, es-toolkit) · ❌ (Remeda, Radashi, Ramda, Effect, Modern Dash, Antfu)

Example​

let attempts = 0;
const saveData = after(() => {
  attempts++;
  console.log(`Data saved! Attempt #${attempts}`);
}, 3);

saveData(); // Nothing happens
saveData(); // Nothing happens
saveData(); // "Data saved! Attempt #1"
saveData(); // "Data saved! Attempt #2"
saveData(); // "Data saved! Attempt #3"

// Use case: Only start logging after initial setup calls
const logger = after((message: string) => {
  console.log(`[LOG] ${message}`);
}, 2);

logger('Initializing...'); // Nothing (setup call)
logger('Loading config...'); // "[LOG] Loading config..."
logger('Ready!'); // "[LOG] Ready!"

How it works?​

Creates a function that only invokes after being called n times. The first n-1 calls return undefined — starting from call n, the function executes normally.

Visual Timeline​

Use Cases​

Trigger after warmup period 📌​

Execute only after a certain number of preliminary calls have occurred. Perfect for waiting until a system stabilizes or data accumulates.

// Only start calculating average after 5 data points
const updateAverage = after(5, (values) => {
  const avg = values.reduce((a, b) => a + b, 0) / values.length;
  displayAverage(avg);
});

sensor.on("reading", (value) => {
  readings.push(value);
  updateAverage(readings);
});
// First 4 readings: nothing happens
// 5th reading onward: average is displayed

Confirm after multiple signals 📌​

Require multiple confirmations before executing a destructive action. Essential for safety mechanisms and preventing accidental triggers.

const confirmDelete = after(2, () => {
  deleteAllData();
  console.log("Data deleted after double confirmation");
});

// User must click "Delete" twice
deleteButton.on("click", confirmDelete);

Batch notifications after threshold 📌​

Send a summary notification only after N events accumulate. Useful for reducing notification spam while still alerting users.

const notifyBatch = after(10, (errors) => {
  sendAlert(`${errors.length} errors occurred in the last hour`);
});

errorStream.on("error", (err) => {
  errorLog.push(err);
  notifyBatch(errorLog);
});
// Alert sent only after 10+ errors