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?

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!

You've created an implicit global variable in a non-strict context. — Jackson, Apr 06 '16 at 07:55
If you are in strict mode (which i would argue you always should be) that would throw an exception and not create an implicit global. Also that isn't ES6 related, that behavior was present before ES6. — ste2425, Apr 06 '16 at 07:58
Seems some minifers benefit from dropping those characters but otherwise use the designated keyword — Ben, Apr 03 '20 at 12:07
See [also](https://stackoverflow.com/questions/500431/what-is-the-scope-of-variables-in-javascript?rq=1). — Ben Aston, Apr 14 '20 at 20:36

score 14 · Accepted Answer · edited Apr 14 '20 at 20:35

14

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/

edited Apr 14 '20 at 20:35

Ben Aston

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answered Apr 06 '16 at 08:00

xcodebuild

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score 2 · Answer 2 · answered Apr 06 '16 at 07:59

2

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

answered Apr 06 '16 at 07:59

Bálint

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score 1 · Answer 3 · answered Apr 05 '20 at 17:10

1

Adding to the previous two answers:

Assignment without the keyword returns the value:

> const x = 4
undefined
> let y = 4
undefined
> var z = 4
undefined
> a = 4
4

And dropping the keyword is used in some minifiers to cutdown on payload

answered Apr 05 '20 at 17:10

Ben

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score 1 · Answer 4 · edited Aug 28 '21 at 19:03

1

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.

edited Aug 28 '21 at 19:03

Pierre.Vriens

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answered Aug 28 '21 at 05:09

Vinícius Melo

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ES6/JavaScript - declaring variables without let/const

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