1. Function declaration
Binary Search expects a sorted array and a target value.
function binarySearch(arr, target)Typorithm · medium
Binary Search
▷ Interactive algorithm labsearchbinary-searchboundarieslogarithmic-time
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Step 1 of 8 · intro card
Learn Binary Search
Search Algorithms
Binary Search repeatedly discards half of a sorted search range.
Binary Search only works when the array is sorted.
It checks the middle value, then decides which half can be ignored.
The left and right boundaries move inward until the target is found or the range is empty.
Preparing algorithm lab…
01
Use left and right boundaries.
02
Check while left <= right.
03
Use Math.floor for the middle index.
04
Move with mid + 1 and mid - 1.
function binarySearch(arr, target) {
let left = 0;
let right = arr.length - 1;
while (left <= right) {
const mid = Math.floor((left + right) / 2);
if (arr[mid] === target) return mid;
if (arr[mid] < target) left = mid + 1;
else right = mid - 1;
}
return -1;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Left boundary
left starts at the first searchable index.
let left = 03. Right boundary
right starts at the last valid index.
let right = arr.length - 14. Inclusive range
Use <= so a one-item search range is still checked.
while (left <= right)5. Middle index
Math.floor turns the midpoint calculation into a valid integer index.
Math.floor((left + right) / 2)6. Return index
When the middle value matches target, return the middle index.
if (arr[mid] === target) return mid7. Discard left half
If middle is too small, discard it and everything to its left.
left = mid + 18. Discard right half
If middle is too large, discard it and everything to its right.
right = mid - 19. Not found
Return -1 after the search range becomes empty.
return -1A complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.