Index of array may be array by itself (tested in Chrome):
a = [1, 2, 3]
index = [1]
a[index] // returns 2
Is there any official documentation of this behavior?
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1but what if `index = ['hello world']` or `[1,2]`? – Jaromanda X Nov 15 '17 at 03:08
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3The subscript operator converts its argument to a string, and the `.toString()` of an array is `.join()` – 4castle Nov 15 '17 at 03:10
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1`a[1]` is `a['1']` is `a[['1'].join(',')]` is `a[String(['1'])]` is `a[['1']]` is `a[[1]]`. – Ry- Nov 15 '17 at 03:11
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1Try this. What result do you get? `var a = [1, 2, 3] var index = [1, 2] console.log(a[index]);` – Gary Nov 15 '17 at 03:12
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This is one of the bad parts. – Aluan Haddad Nov 15 '17 at 03:18
3 Answers
4
Is there any official documentation of this behavior?
12.3.2.1Runtime Semantics: Evaluation
defines the following 3 steps
3 Let propertyNameReference be the result of evaluating Expression.
4 Let propertyNameValue be ? GetValue(propertyNameReference).
6 Let propertyKey be ? ToPropertyKey(propertyNameValue).
Then 7.1.14ToPropertyKey ( argument ) is defined as
- Let
keybe ?ToPrimitive(argument, hint String). - If
Type(key)isSymbol, then
a. Returnkey. - Return !
ToString(key).
Which effectively means, that unless the expression returns a Symbol - it (the key) would be converted to a string.
zerkms
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As @Ryan and @4castle mentioned, javascript convert key(inside []) to string using [].join(',').
You can test it in this snippet.
var abc = {
"1,2": "ddd"
};
console.log(abc[[1,2]]);
HerrGanzorig
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a[index] will do a toString on the index, and since index is an array, its to string is index.join() making the output a['1']
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Property_Accessors
Davi DeBarros
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