var newArr = (newArr = oldArr.slice(), newArr.splice(newArr.indexOf(item), 1), newArr);

By combining JavaScript’s comma operator, Array slice and Array splice, we can conditionally duplicate JavaScript arrays.

Example code:

// Removing '3' from the new Array.
var a = [1, 2, 3, 4, 5];
var b = (b = a.slice(), b.splice(2,1), b);
// b = [1, 2, 4, 5];

Here we have to use a.slice() to copy Array a to b. Otherwise changes made to b would reflect on a as well.

Further Reading

• The JavaScript Comma Operator