CODEWITHSUNDEEP
javascript
Combine
Combine

Reputation: 4214

ES6/JavaScript - declaring variables without let/const

In JavaScript / ES6.

What happens when you have something like:

x = 5;

console.log(x);  // 5

Is the interpreter automatically adding let at runtime? Why is this working without errors?

Upvotes: 26

Views: 14001

Answers (4)

Vinícius Melo
Vinícius Melo

Reputation: 81

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.

Upvotes: 1

xcodebuild
xcodebuild

Reputation: 1221

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/

Upvotes: 16

Ben
Ben

Reputation: 3654

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

Upvotes: 1

Bálint
Bálint

Reputation: 4039

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

Upvotes: 2

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