unscopables-mutation.js

Enable keyboard shortcuts

// When obj[@@unscopables].x changes, bindings appear and disappear accordingly.

let x = "global";

function getX() { return x; }

let unscopables = {x: true};

let obj = {x: "obj", [Symbol.unscopables]: unscopables};

with (obj) {

    assertEq(x, "global");

    x = "global-1";

    assertEq(x, "global-1");

    assertEq(obj.x, "obj");

    unscopables.x = false;  // suddenly x appears in the with-environment

    assertEq(x, "obj");

    x = "obj-1";

    assertEq(getX(), "global-1");  // unchanged

    assertEq(obj.x, "obj-1");

    unscopables.x = true;  // *poof*

    assertEq(x, "global-1");

    x = "global-2";

    assertEq(getX(), "global-2");

    assertEq(obj.x, "obj-1");  // unchanged

    // The determination of which binding is assigned happens when the LHS of

    // assignment is evaluated, before the RHS. This is observable if we make

    // the binding appear or disappear during evaluation of the RHS, before

    // assigning.

    x = (unscopables.x = false, "global-3");

    assertEq(getX(), "global-3");

    assertEq(obj.x, "obj-1");

    x = (unscopables.x = true, "obj-2");

    assertEq(getX(), "global-3");

    assertEq(obj.x, "obj-2");

assertEq(x, "global-3");

reportCompare(0, 0);