search first index with Binary Search(possible duplicate array)

Question

This is a variation of 37. implement Binary Search (unique).

Your are given a sorted ascending array of number, but might have duplicates, you are asked to return the first index of a target number.

If not found return -1.

note

Please don't use Array.prototype.indexOf(), it is not our goal.

Code

/**
 * @param {number[]} arr - ascending array with duplicates
 * @param {number} target
 * @return {number}
 */
function firstIndex(arr, target){
  // your code here
}

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