无论括号的数量如何，通过括号表示法访问 object 属性

Question

An object's properties can be accessed through bracket notation by doing the following:

let obj = {a: "test"}

obj["a"]

However, I was not aware that the same object's property could also be accessed by doing:

let v = ["a"] // An array

obj[v]

or

obj[["a"]]

or

obj[[[[[[[[[["a"]]]]]]]]]]

Just wondering, why is that?

I stumbled on this behaviour after storing an array into a variable and mistakingly using the variable/array, rather than the first item of the array, to access an object's property and surprisingly... it didn't throw an error but returned the value.

Answer 1

All object keys are strings. When you use bracket notation foo[bar] the variable you try to fetch will be converted to a string:

 const bar = { toString() { return "hello"; } } const foo = { hello: "world" } console.log(foo[bar]);

When arrays are converted to a string, join(",") is implicitly called on them. And if an array has a single value, the result is the single value as a string:

 const arr = ["hello"]; console.log(arr.toString()); console.log(String(arr)); console.log(arr.join(","));

If you have nested arrays, each with one item, you'd still get a single string out of the conversion, since join() also converts all the members into strings, so with String([["hi"]]) you (roughly) get:

[["hi"]].join(",") -> String(["hi"]) -> ["hi"].join(",") -> String("hi")

So, if you supply an array as a key, it works, as long as you only have a single value in each array:

 const foo = { hello: "world" }; const arr = [[["hello"]]]; console.log(foo[arr]); console.log(foo[String(arr)]); console.log(foo[arr.toString()]); console.log(foo[arr.join(",")]);

Answer 2

console.log(['a'].toString()); // "a"

It's because a key needs to be a string and javascript does type coercion, converting the array to a string automatically.