Pacerier Pacerier - 4 months ago 21
Javascript Question

Keys in Javascript objects can only be strings?

jshashtable states:


JavaScript's built-in objects do provide hashtable functionality using
the square brackets notation for
properties, provided your keys are
strings or numbers:


From what I know, keys are only strings, (since numbers are coerced into strings anyway). I just want to check and be sure that what is stated above is false (since keys can't be numbers).

Did ECMA standard stated anything about this..

Or is the implementation browser-specific?

Answer

JavaScript's built-in objects do provide hashtable functionality using the square brackets notation for properties, provided your keys are strings or numbers

That seems to be incorrect - object keys are always strings. But that is a different topic to square bracket property access.

See ECMA-262 ยง 11.2.1 (please excuse the bad formatting, StackOverflow isn't very good at this):

Properties are accessed by name, using either the dot notation:

MemberExpression . IdentifierName

CallExpression . IdentifierName

or the bracket notation:

MemberExpression [ Expression ]

CallExpression [ Expression ]

The dot notation is explained by the following syntactic conversion:

MemberExpression . IdentifierName

is identical in its behaviour to

MemberExpression [ <identifier-name-string> ]

and similarly

CallExpression . IdentifierName

is identical in its behaviour to

CallExpression [ <identifier-name-string> ]

where <identifier-name-string> is a string literal containing the same sequence of characters after processing of Unicode escape sequences as the IdentifierName.

So when using dot notation, the bit after the dot must fit the criteria fro an IdentifierName. But when using square brackets, an expression is provided that is evaluated and resolved to a string.

Briefly, square bracket notation is provided so that properties can be accessed using an expression, e.g.

var y = {};
var x = 'foo';
y[x] = 'foo value';

In the above, x is provided in square brackets so it is evalueated, returning the stirng 'foo'. Since this property doesn't exist on y yet, it is added. The foo property of y is then assigned a value of 'foo value'.

In general terms, the expression in the square brackets is evaluated and its toString() method called. It is that value that is used as the property name.

In the dot proerty access method, the identifier is not evaluated, so:

y.bar = 'bar value';

creates a property bar with a value bar value.

If you want to create a numeric property, then:

y[5] = 5;

will evaluate 5, see it's not a string, call (more or less) Number(5).toString() which returns the string 5, which is used for the property name. It is then assigned the value 5, which is a number.