Array.push return pushed value?

See my detailed answer here

TLDR;
You can get the return value of the mutated array, when you instead add an element using array.concat[].

concat is a way of "adding" or "joining" two arrays together. The awesome thing about this method, is that it has a return value of the resultant array, so it can be chained.

newArray = oldArray.concat[newItem];

This also allows you to chain functions together

updatedArray = oldArray.filter((item) => { item.id !== updatedItem.id).concat[updatedItem]};

Where item = {id: someID, value: someUpdatedValue}

The main thing to notice is, that you need to pass an array to concat.
So make sure that you put your value to be "pushed" inside a couple of square brackets, and you're good to go.
This will give you the functionality you expected from push()

You can use the + operator to "add" two arrays together, or by passing the arrays to join as parameters to concat().

let arrayAB = arrayA + arrayB;
let arrayCD = concat(arrayC, arrayD);

Note that by using the concat method, you can take advantage of "chaining" commands before and after concat.

Are there any substantial reasons why modifying Array.push() to return the object pushed rather than the length of the new array might be a bad idea?

Of course there is one: Other code will expect Array::push to behave as defined in the specification, i.e. to return the new length. And other developers will find your code incomprehensible if you did redefine builtin functions to behave unexpectedly.

At least choose a different name for the method.

You would then be able to do something like this: someFunction(value, someArray.push({}));

Uh, what? Yeah, my second point already strikes :-)

However, even if you didn't use push this does not get across what you want to do. The composition that you should express is "add an object which consist of a key and a value to an array". With a more functional style, let someFunction return this object, and you can write

var someArray = [],
    value = "hello world";

function someFunction(value, obj) {
    obj["someKey"] = value;
    return obj;
}

someArray.push(someFunction(value, {}));