Suppose I have an object
obj = {
a : 1
}
I'm able to access property a via obj["a"] but I'm also able to access it via obj[["a"]]. How is that possible?
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1Related: Keys in Javascript objects can only be strings?, Using Object as key results always in value of last assignment.Sebastian Simon– Sebastian Simon2019-11-16 12:38:28 +00:00Commented Nov 16, 2019 at 12:38
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1 Answer
Object keys are always strings (or, rarely, symbols). When you do
obj[<expression>]
the interpreter will try to turn expression into a valid key, if it isn't one already. In this case, turning ["a"] into a string results in "a", so both obj["a"] and obj[["a"]] work.
(When an array is implicitly turned into a primitive, like here, it gets .joined by a comma, and ["a"].join(',') === "a")
answered Nov 16, 2019 at 12:11
CertainPerformance
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3 Comments
Clepsyd
I had never realised JS was casting types as deep as it can in nested arrays:
[[[["a"]]]] == "a" evaluates to true
yogupta
While we are on it, why does JS do it this way?
CertainPerformance
@YogeshGupta As opposed to what, throwing an error? Some of it has to do with JS being loosely typed - the interpreter may try to do type conversions automatically to get to a result without throwing. It's kind of confusing, which is why I prefer Typescript, which will forbid such implicit casting