Function arrLastIndexOf

  • arrLastIndexOf<T>(theArray, searchElement, fromIndex?): number

  • The arrLastIndexOf() method returns the last index at which a given element can be found in the array, or -1 if it is not present. arrLastIndexOf() compares searchElement to elements of the Array using strict equality (the same method used by the === or triple-equals operator). NaN values are never compared as equal, so arrLastIndexOf() always returns -1 when searchElement is NaN.

    The arrLastIndexOf() method skips empty slots in sparse arrays.

    The arrLastIndexOf() method is generic. It only expects the this value to have a length property and integer-keyed properties.

    Type Parameters

    • T

      Identifies the type of array elements

    Parameters

    • theArray: ArrayLike<T>

      The array or array like object of elements to be searched.

    • searchElement: T

      The element to locate in the array.

    • Optional fromIndex: number

      Zero-based index at which to start searching backwards, converted to an integer.

      • Negative index counts back from the end of the array — if fromIndex < 0, fromIndex + array.length is used.
      • If fromIndex < -array.length, the array is not searched and -1 is returned. You can think of it conceptually as starting at a nonexistent position before the beginning of the array and going backwards from there. There are no array elements on the way, so searchElement is never found.
      • If fromIndex >= array.length or fromIndex is omitted, array.length - 1 is used, causing the entire array to be searched. You can think of it conceptually as starting at a nonexistent position beyond the end of the array and going backwards from there. It eventually reaches the real end position of the array, at which point it starts searching backwards through the actual array elements.

    Returns number

    The first index of the element in the array; -1 if not found.

    Since

    0.8.0

    Example

    const numbers = [2, 5, 9, 2];
    arrLastIndexOf(numbers, 2); // 3
    arrLastIndexOf(numbers, 7); // -1
    arrLastIndexOf(numbers, 2, 3); // 3
    arrLastIndexOf(numbers, 2, 2); // 0
    arrLastIndexOf(numbers, 2, -2); // 0
    arrLastIndexOf(numbers, 2, -1); // 3

    let indices: number[] = [];
    const array = ["a", "b", "a", "c", "a", "d"];
    const element = "a";
    let idx = arrLastIndexOf(array, element);
    while (idx !== -1) {
      indices.push(idx);
      idx = arrLastIndexOf(array, element, idx ? idx - 1 : -(array.length + 1));
    }
    console.log(indices);
    // [4, 2, 0]

    function updateVegetablesCollection (veggies, veggie) {
        if (arrLastIndexOf(veggies, veggie) === -1) {
            veggies.push(veggie);
            console.log('New veggies collection is : ' + veggies);
        } else {
            console.log(veggie + ' already exists in the veggies collection.');
        }
    }

    let veggies = ['potato', 'tomato', 'chillies', 'green-pepper'];

    updateVegetablesCollection(veggies, 'spinach');
    // New veggies collection is : potato,tomato,chillies,green-pepper,spinach
    updateVegetablesCollection(veggies, 'spinach');
    // spinach already exists in the veggies collection.

    // Array Like
    let arrayLike = {
      length: 3,
      0: "potato",
      1: "tomato",
      2: "chillies",
      3: "green-pepper"  // Not checked as index is > length
    };

    arrLastIndexOf(arrayLike, "potato"); // 0
    arrLastIndexOf(arrayLike, "tomato"); // 1
    arrLastIndexOf(arrayLike, "chillies"); 2
    arrLastIndexOf(arrayLike, "green-pepper"); // -1

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