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function indexFirstLast(target, arr) {
    let [first, last] = [-1, -1];
    for (let i = arr.length - 1; i >= 0; i--) {
        if (arr[i] === target) {
            last = i;
            break;
        }
    }
    for (let i = 0; i <= last; i++) {
        if (arr[i] === target) {
            first = i;
            break;
        }
    }
    return [first, last];
}

Here is a revised, simpler version of your code.

function indexFirstLast(target, arr) {
    let [first, last] = [-1, -1];
    for (let i = 0; i < arr.length && arr[i] <= target; i++) {
        if (arr[i] === target) {
            if (first === -1) {
                first = i;
            }
            last = i;
        }
    }
    return [first, last];
}

A simple, readable version.

function indexFirstLast(target, arr) {
    let [first, last] = [-1, -1];
    for (let i = arr.length - 1; i >= 0; i--) {
        if (arr[i] === target) {
            last = i;
            break;
        }
    }
    for (let i = 0; i <= last; i++) {
        if (arr[i] === target) {
            first = i;
            break;
        }
    }
    return [first, last];
}

function indexFirstLast(target, arr) {
    let [first, last] = [-1, -1];
    for (let i = arr.length - 1; i >= 0; i--) {
        if (arr[i] === target) {
            last = i;
            break;
        }
    }
    for (let i = 0; i <= last; i++) {
        if (arr[i] === target) {
            first = i;
            break;
        }
    }
    return [first, last];
}

A simple, readable version.

function indexFirstLast(target, arr) {
    let [first, last] = [-1, -1];
    for (let i = arr.length - 1; i >= 0; i--) {
        if (arr[i] === target) {
            last = i;
            break
        }
    }
    for (let i = 0; i <= last; i++) {
        if (arr[i] === target) {
            first = i;
            break
        }
    }
    return [first, last];
}

A simple, readable version.

function indexFirstLast(target, arr) {
    let [first, last] = [-1, -1];
    for (let i = arr.length - 1; i >= 0; i--) {
        if (arr[i] === target) {
            last = i;
            break;
        }
    }
    for (let i = 0; i <= last; i++) {
        if (arr[i] === target) {
            first = i;
            break;
        }
    }
    return [first, last];
}