What's the output?

Question

```javascript
(() => {
  let x = (y = 10);
})();

console.log(typeof x);
console.log(typeof y);
```

Accepted Answer

`let x = (y = 10);` is actually shorthand for:

```javascript
y = 10;
let x = y;
```

When we set `y` equal to `10`, we actually add a property `y` to the global object (`window` in the browser, `global` in Node). In a browser, `window.y` is now equal to `10`.

Then, we declare a variable `x` with the value of `y`, which is `10`. Variables declared with the `let` keyword are _block scoped_, they are only defined within the block they're declared in; the immediately invoked function expression (IIFE) in this case. When we use the `typeof` operator, the operand `x` is not defined: we are trying to access `x` outside of the block it's declared in. This means that `x` is not defined. Values who haven't been assigned a value or declared are of type `"undefined"`. `console.log(typeof x)` returns `"undefined"`.

However, we created a global variable `y` when setting `y` equal to `10`. This value is accessible anywhere in our code. `y` is defined, and holds a value of type `"number"`. `console.log(typeof y)` returns `"number"`.

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