Make immutable pop(), push(), shift(), unshift()?

#javascript #webdev #discuss

We know very well how these functions work in Javascript. So, here I am going to write immutable implementations for the same functions as per my understanding. Please do comment your suggestions/concerns/questions. I will always happy to update this article.

function pop(arr) {
    let newArr = [...arr];
    newArr.length = arr.length - 1;
    return newArr;
}

function push(arr, ...item) {
    const newArr = [...arr];
    if (item.length >= 1) {
        for (let i = 0; i < item.length; i++) {
            newArr[newArr.length] = item[i];
        }
    }
    return newArr;
}

function shift(arr) {
    let [a, ...b] = arr;
    return b;
}

function unshift(arr, ...item) {
    const newArr = [];
    if (item.length >= 1) {
        for (let i = 0; i < item.length; i++) {
            newArr[i] = item[i];
        }
    }
    newArr.push(...arr);
    return newArr;
}

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Top comments (3)

Tracy Gilmore • Mar 15 '22 • Edited

Hi Vishwak,
Your implementation of shift has a critical flaw. The variables a and b should have been declared as local variables instead of adding to the global scope. Otherwise the function was a good cue for how the other functions can be written more concisely as follows.

    function pop(arr) {
        return arr.slice(0, Math.max(0, arr.length - 2));
    }

    function push(arr, ...items) {
        return [...arr, ...items];
    }

    function shift(arr) {
        return arr.slice(1, Math.max(0, arr.length));
    }

    function unshift(arr, ...items) {
        return [...items, ...arr];
    }

Kurapati Mahesh • Mar 16 '22 • Edited

You have written in declarative style.

Also, your pop can't work as expected. And, does Math.max required?

I followed little imperative.

Thanks. I will update for shift.

Tracy Gilmore • Mar 16 '22

Vishwak, The Math.max guards against the deduction of length going negative but you are correct there was an error. These are better.

    function pop(arr) {
        return arr.slice(0, Math.max(0, arr.length - 1));
    }

    function push(arr, ...items) {
        return [...arr, ...items];
    }

    function shift(arr) {
        return arr.slice(1, arr.length);
    }

    function unshift(arr, ...items) {
        return [...items, ...arr];
    }

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Make immutable pop(), push(), shift(), unshift()?

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