Demo of how foreign keys are not required to link objects in Python and JavaScript because of Object references.

NoForeignKey.js

var x, y, Xs, Ys;

// Assign objects.
x = {
    "id": 0,
    "Ys": []
};

y = {
    "id": 0,
    "Xs": []
};

// Add references.
y.Xs.push(x);
y.Xs[0].Ys.push(y);

// Remove references (reverse the addition).
Xs = y.Xs
Ys = Xs[Xs.indexOf(x)].Ys
Ys.splice(Ys.indexOf(y), 1);
Xs.splice(Xs.indexOf(x), 1);

NoForeignKey.py

# Assign objects.
x = {
    "id": 0,
    "Ys": []
}

y = {
    "id": 0,
    "Xs": []
}

# Add references.
y["Xs"].append(x)
y["Xs"][0]["Ys"].append(y)

# Remove references (reverse the addition).
Xs = y["Xs"]
Ys = Xs[Xs.index(x)]["Ys"]
del Ys[Ys.index(y)]
del Xs[Xs.index(x)]

NoForeignKey2.js

// This solution reduces the number of globals using an IIFE.
var x, y;

// Assign objects.
x = {
    "id": 0,
    "Ys": []
};

y = {
    "id": 0,
    "Xs": []
};

// Add references.
y.Xs.push(x);
y.Xs[0].Ys.push(y);

// Remove references (reverse the addition).
(function (x, y) {
    var Xs = y.Xs,
        Ys = Xs[Xs.indexOf(x)].Ys;

    Ys.splice(Ys.indexOf(y), 1);
    Xs.splice(Xs.indexOf(x), 1);
}(x, y));