ladybird/Tests/LibJS/Runtime/assignment-evaluation-order.js

test("Assignment should always evaluate LHS first", () => {
    function go(a) {
        let i = 0;
        a[i] = a[++i];
    }

    let a = [1, 2, 3];
    go(a);
    expect(a).toEqual([2, 2, 3]);
});

test("Binary assignment should always evaluate LHS first", () => {
    function go(a) {
        let i = 0;
        a[i] |= a[++i];
    }

    let a = [1, 2];
    go(a);
    expect(a).toEqual([3, 2]);
});

test("Base object of lhs of assignment is copied to preserve evaluation order", () => {
    let topLevel = {};
    function go() {
        let temp = topLevel;
        temp.test = temp = temp.test || {};
    }

    go();
    expect(topLevel.test).not.toBeUndefined();
    expect(topLevel.test).toEqual({});
});