Get comprehensive list of all existing built-in variables in browser

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I want to let the user define certain variables without overriding existing ones. For example, an allowed new variable:

myJSON = 123

And an invalid one:

JSON = 123

Because it overrides the existing JSON object. Before adding I could simply check if the variable is different from undefined. But I would prefer to show the user which variables are already taken too.

How can I get a comprehensive list of the browser's built-in variables?

Note that Object.keys(window) and Object.keys(document) don't contain JSON. Which other places should I check to get a comprehensive list of taken variables?

browser variables javascript
1 Answers

So, originally, I thought that the limitation was that Object.keys enumerates only an object's own properties and not inherited ones. However, trying a simple for...in loop without a hasOwnProperty check failed to yield much more than Object.keys. I also tried looping over globalThis to no avail.

A bit of further research showed that JSON is part of the family of "Standard Built-In Objects". When researching that, I came upon a similar Stack Overflow question regarding how to list all standard built-in objects in Node, and the solution was to leverage getOwnPropertyNames. This proved to be the solution-- rather than getting ≈300 results, I instead got 1000+, among which were "JSON" and "Error".

const allWindowProperties = Object.getOwnPropertyNames(window);
console.log(allWindowProperties);
console.log(allWindowProperties.includes('JSON'));
console.log(allWindowProperties.includes('Error'));

So hopefully this solves your problem. My original comment, however, is still worth exploring: How has this use case manifested? Why is it a necessity to allow the user direct access to the runtime environment to declare variables at all, let alone in the global namespace? Perhaps you have a good reason, and I do think this is an interesting question, but I wonder if perhaps we have a classic X-Y problem here.

user_68313410
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