Question 304025 in Javascript Question posted in
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Javascript – I am trying to modify an array given as argument in a function by reversing its elements

i expect the elements in arr be reversed, but it still log as [‘a’,’b’,’c’,’d’,’e’]

i think i am doing some fundamental problem but i cannot figure out what it is

    function reverseArray(array) {
       let array02 = [];
       let len = array.length-1;
       for (let i=0; i<=len; i++){
           array02[len-i] = array[i];
       }
    array = array02;
    return array;
    }
    var arr = ['a','b','c','d','e'];
    reverseArray(arr);
    console.log(arr);
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4

Answers


  1. 0

    Yes, the fundamental point you’re missing is that you’re doing basically this:

    function reverseArray(array) {
    array = ....some new data
}

var arr = ...something
reverseArray(arr)

    and expect the variable arr to change its value. This doesn’t work in Javascript, because Javascript passes arguments by value and any assignment to a function argument (array = ....) cannot affect a variable in the calling code.

    What you need here is either return a new value and assign it in the caller code

    function reverseArray(array) {
    return ....some new data
}

var arr = ...something
var newArr = reverseArray(arr)

    or change the value itself, and not the variable:

    function reverseArray(array) {
    array[index] =  ....some new data
}

var arr = ...something
reverseArray(arr)
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  2. 0

    You can modify the function to directly reverse the elements of the input array without creating a new array like this:-

    function reverseArray(array) {
   let len = array.length - 1;
   for (let i = 0; i <= len / 2; i++) {
       // Swap elements at positions i and len - i
       let temp = array[i];
       array[i] = array[len - i];
       array[len - i] = temp;
   }
}

var arr = ['a', 'b', 'c', 'd', 'e'];
reverseArray(arr);
console.log(arr);
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  3. 0

    You are re-assign array to array02 inside the function. That is why main array is lost. Try this one for reversing array.

    You can also use array.reverse function.

    function reverseArray(array) {
    array.reverse();
    return array;
}

var arr = ['a', 'b', 'c', 'd', 'e'];
reverseArray(arr);
console.log(arr);

    Or like this way.

    function reverseArray(array) {
    let lenth = array.length - 1;
    for (let i = 0; i <= lenth / 2; i++) {
        let tmp = array[i];
        array[i] = array[lenth - i];
        array[lenth - i] = tmp;
    }
    return array;
}

var arr = ['a', 'b', 'c', 'd', 'e'];
reverseArray(arr);
console.log(arr);
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  4. 0

    What you are doing is assigning array1 to array2 inside reverseArray(array) function. By doing so you are losing the original array.

    function reverseArray(array) {
   let len = array.length - 1;
   for (let i = 0; i <= len / 2; i++) {           
       let temp = array[i]; // Swap elements using temp variable
       array[i] = array[len - i];
       array[len - i] = temp;
   }
}

var arr = ['a', 'b', 'c', 'd', 'e'];
console.log(JSON.stringify(arr))
reverseArray(arr);
console.log(JSON.stringify(arr));
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