Binary Search First Occurrence

Find the first copy of a duplicated target by recording matches and continuing to search the left half.

Algorithm

Basic Implementation

basic.js

const arr = [1, 2, 4, 4, 4, 7, 9];
const target = 4;
let lo = 0;
let hi = arr.length - 1;
let result = -1;
while (lo <= hi) {
  const mid = lo + Math.floor((hi - lo) / 2);
  if (arr[mid] === target) {
    result = mid;
    hi = mid - 1;
  } else if (arr[mid] < target) {
    lo = mid + 1;
  } else {
    hi = mid - 1;
  }
}
console.log(result);

Complexity

Time: O(log n)
Space: O(1)

Implementation notes

Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
The final output is intentionally small and deterministic for cross-language comparison.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-first`.

cross-language comparison This JavaScript DSA version keeps the same data and final output as every other DSA book in this wave.