ES6/JavaScript - declaring variables without let/const

Question

Ok, my question is simple: In JavaScript / ES6

what happens when you have something like

 x = 5;

 console.log(x);  // 5

is the interpreter automatically adding "let" at runtime or why is this working without errors?

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Edit: Strict Mode The syntax of ES5 allowed for something called implicit globals, which have been the source of many frustrating programming errors. In short, if you forgot to declare a variable with var , JavaScript would merrily assume you were referring to a global variable. If no such global variable existed, it would create one! You can imagine the problems this caused.

I see. Thank you for all comments. I now understand why is this happening. Thanks!

Answer 1

By omitting let, const, or var in non-strict mode, you create a property on the global object.

By the way, babel will add "use strict"; by default. So you will get a error with babel.

You can try it here: https://babeljs.io/repl/

Answer 2

In this case, x becomes a global variable. Try to avoid this as global variables can be hard on the browser.

Answer 3

then you're not declaring/defining a variable; instead you're creating/defining (or overwriting) some global/window obj property

x = 5;

is the same as

window.x = 5;

You can check it with window, or with the this keyword in your browser console; look at the global/window obj properties, you'll see the x property there.