You need to use assignment separate from declaration syntax:

({
    screenings,
    size
} = source);

Babel REPL Example

From the linked docs:

The ( .. ) around the assignment statement is required syntax when using object literal destructuring assignment without a declaration

And obviously you need to use this as you can't redeclare a let variable. If you were using var, you could just redeclare var { screenings, size } = source;

You can do this with array destructuring, i.e.:

const complexPoint = [1,2];

let x, y;
[x,y] = complexPoint;

As for object destructuring, an equivalent syntax would not work as it would throw off the interpreter:

const complexPoint = {x:1,y:2};

let x, y;
{x,y} = complexPoint; // THIS WOULD NOT WORK

A workaround could be:

const complexPoint = {x:1,y:2};

let x, y;
[x,y] = [complexPoint.x, complexPoint.y];

// Or
[x,y] = Object.values(complexPoint);

UPDATE:

It appears you can destructure an object into existing variables by wrapping the assignment in parenthesis and turning it into an expression. So this should work:

const complexPoint = {x:1,y:2};

let x, y;
({x,y} = complexPoint); // THIS WILL WORK

While ugly and a bit repetitive, you can do

({x: oof.x, y: oof.y} = foo);

which will read the two values of the foo object, and write them to their respective locations on the oof object.

Personally I'd still rather read

oof.x = foo.x;
oof.y = foo.y;

or

['x', 'y'].forEach(prop => oof[prop] = foo[prop]);

though.

When you are destructuring an Object,

1. you need to use the same variable names as the keys in the object. Only then you will get one to one correspondence and the values will be destructured properly.

2. and you need to wrap the entire assignment in parenthesis if you are not using declaration statement, otherwise the object literal in the left hand side expression would be considered as a block and you will get syntax error.

So your fixed code would look like this

'use strict';
let cat, dog, mouse;
let obj = {cat: 'meow', dog: 'woof', mouse: 'squeak'};
({cat, dog, mouse} = obj);     // Note the `()` around

which is equivalent to

'use strict';
let obj = {cat: 'meow', dog: 'woof', mouse: 'squeak'};
let {cat, dog, mouse} = obj;