_.debounce leading and trailing configuration change in lodash 4.x

Answer

With {leading:true, trailing:true} a debounced function invoked twice within the wait window produces exactly two calls: the leading‑edge call from the first invocation and a trailing‑edge call that fires after the wait period following the second invocation. If the function is invoked only once, you still get two calls – an immediate leading call and a guaranteed trailing call after the wait.

Explanation

The leading edge runs immediately on the first call and starts a trailing timer. Each subsequent call resets that timer; if another call occurs before the timer expires, the pending trailing invocation is cancelled and a new timer begins from that call. Therefore:

• Two calls spaced less than wait → leading from call 1, trailing from call 2.
• One call → leading immediately, trailing after wait (no cancellation).

When migrating from a leading‑only or trailing‑only debounce to the combined configuration, note that the leading edge can cancel a pending trailing call that would have occurred in the pure trailing‑only case. This can reduce the number of trailing invocations observed for bursty call patterns.

Verification steps

1. Install lodash 4.x (e.g., npm i lodash@4.17.21).
2. Create a test script:

const _ = require('lodash');
let count = 0;
const debounced = _.debounce(() => { console.log(Date.now(), ++count); }, 100, {leading:true, trailing:true});
// Scenario A: two calls within wait
debounced();
setTimeout(() => debounced(), 50);
setTimeout(() => { console.log('--- end A ---'); }, 200);
// Scenario B: single call
debounced();
setTimeout(() => { console.log('--- end B ---'); }, 200);

3. Run the script with Node and confirm the console output shows two timestamps for scenario A and two timestamps for scenario B.