How to toggle an element in array using JavaScript?

You could do it without a 3rd party library, this would be more efficient, like this. (this only removes the first instance of a value if found, not multiple)

Javascript

var a = [0, 1, 2, 3, 4, 6, 7, 8, 9],
    b = 5,
    c = 6;

function addOrRemove(array, value) {
    var index = array.indexOf(value);

    if (index === -1) {
        array.push(value);
    } else {
        array.splice(index, 1);
    }
}

console.log(a);

addOrRemove(a, b);
console.log(a);

addOrRemove(a, c);
console.log(a);

Output

[0, 1, 2, 3, 4, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 6, 7, 8, 9, 5]
[0, 1, 2, 3, 4, 7, 8, 9, 5] 

On jsfiddle

You can use the lodash function "xor":

_.xor([2, 1], [2, 3]);
// => [1, 3]

If you don't have an array as 2nd parameter you can simpy wrap the variable into an array

var variableToInsertOrRemove = 2;
_.xor([2, 1], [variableToInsertOrRemove]);
// => [1]
_.xor([1, 3], [variableToInsertOrRemove]);
// => [1, 2, 3]

Here's the doc: https://lodash.com/docs/4.16.4#xor

For immutable state ( clone array ):

const addOrRemove = (arr, item) => arr.includes(item) ? arr.filter(i => i !== item) : [ ...arr, item ];

see also: Remove properties from objects (JavaScript)