8000 adding `{}[] ` returns array section by hsl0 · Pull Request #165 · denysdovhan/wtfjs · GitHub
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adding {}[] returns array section #165

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hsl0
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@hsl0 hsl0 commented Oct 27, 2020 •
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I found this simple strangeness of javascript!

I have tested it on Chromium Edge, Chrome, Firefox and IE.
In node.js, {}[] returns array, but {foo: 'bar'}['foo'] returns 'bar'

@libook
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libook commented Oct 28, 2020 •
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a={foo: 'bar'}['foo'] // a='bar'
a={}[] // Uncaught SyntaxError: Unexpected token ']'
{foo: 'bar'}[] // []
a={foo: 'bar'}[] // Uncaught SyntaxError: Unexpected token ']'

{foo: 'bar'}['foo'] means read the value of 'foo' key from object {foo: 'bar'}.
But if there is no item in the array, they will be one emty object {} and one empty array [] for JS engine. Just like {};[];.

> (()=>({}[]))()
(()=>({}[]))()
         ^

Uncaught SyntaxError: Unexpected token ']'


> (()=>({}['foo']))()
undefined


> (()=>({foo:'bar'}['foo']))()
'bar'


> (()=>({foo:'bar'}[]))()
(()=>({foo:'bar'}[]))()
                  ^

Uncaught SyntaxError: Unexpected token ']'

I did some "return test" with functions. You can see:
It was not {}[] returned []; it was [] returned [].

@hsl0
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hsl0 commented Oct 28, 2020 •
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@libook hadn't mean "{}[] returns array, but {foo: 'bar'}['foo'] returns 'bar' in node.js" is strange, but I meant "{}[] returns array and {foo: 'bar'}['foo'] returns array in browser console" is strange as I wrote in README.

Anyway, thanks for letting me know how this happened! You didn't answer the intended question, but the answer could explain my curiosity.

I want to know one more thing, how {}{} returns undefined in console, instead of returning object or throwing error? Is it strange behavior? (Chromium, node)

{}{}; // undefined
{}{}{}; // undefined
{}{}{}{}; // undefined
{foo: 'bar'}{}; // 'bar'
{}{foo: 'bar'}; // 'bar'
{}{foo: 'bar'}{}; // 'bar'
{a: 'b'}{c:' d'}{}; // 'd'
{a: 'b', c: 'd'}{}; // SyntaxError: Unexpected token ':'
({}{}); // SyntaxError: Unexpected token '{'

@libook
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libook commented Oct 29, 2020

Yes, they are strange.

There are 2 meaning for {}: object and block.
For example, the {} in ()=>{} means block. So we need to use ()=>({}) to return an object.
So there are 2 questions:

  1. For each expression, it may not return one single value . Where did REPL read the final result?
  2. For each {}, is it block or object?

Also, REPL is different with directly executing files.

@hsl0
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hsl0 commented Oct 30, 2020

@libook Then, can it be recorded in WTFJS?

@hsl0 hsl0 changed the title adding {}[] returns array section adding {}{} Oct 30, 2020
@hsl0 hsl0 closed this Oct 30, 2020
@hsl0 hsl0 changed the title adding {}{} adding {}[] returns array section Oct 30, 2020
@hsl0 hsl0 mentioned this pull request Oct 30, 2020
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@denysdovhan denysdovhan added the question Questions from readers label Feb 3, 2021
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@dineshvgp dineshvgp mentioned this pull request May 13, 2022
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@avdkishore avdkishore mentioned this pull request May 14, 2022
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