Keys in Javascript objects can only be strings?

By | July 14, 2018
Questions:

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?

Answers:

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 may be strings or (since ECMAScript 2015, aka ECMA-262 ed 6) symbols. But that is a different topic to square bracket property access.

See ECMA-262 ed 3 § 11.2.1 (Please also see ECMAScript 2017 (draft).):

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 for 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 evaluated, returning the string ‘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 property 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.

Edit

This answer was written when ECMAScript ed3 was current, however things have moved on. Please see later references and MDN.

Questions:
Answers:

You’re right keys can only be strings, and numeric keys such as those used in Arrays are coerced and stored as strings.

var arr = [true];
arr[0] === true;
arr['0'] = false;
arr[0] === false;

ECMAScript spec, page 42: ECMA-262 Script 3rd Edition.

The production PropertyName : NumericLiteral is evaluated as follows:

  1. Form the value of the NumericLiteral.
  2. Return ToString(Result(1)).
Questions:
Answers:

The keys are always strings. This means you can’t use an object instance’s identity as a key.

In Flash’s ActionScript 3 (uses strong run-time types unlike AS2) there is a Dictionary object which uses strict equality comparison for keys, so that you can use object instances themselves as keys (as well as numbers, strings, etc.).

If you wanted to do the same thing in JavaScript, it would be difficult, because you’d have to generate your own unique object ids and attach them to every object you wanted to track. Some have suggested adding a prototype function to the Object class, but that would add overhead to every object unnecessarily. In any case, you’d want to give an object a trackable ID via a function call that assigns an incrementing static number to a unique property such as “__objectid__“.

It would then be conceivable to create a Dictionary-like class with methods like Add(key,value), but it would have to store strings, numbers, and objects in three separate internal hashes to ensure “3” doesn’t collide with the number 3 or the object with id 3. The add method would have to automatically assigned an __objectid__ to any key of type object that didn’t already have an id assigned. Even after all that, you wouldn’t be able to access the dictionary using brackets, unless there are some hooks for property assignments that I’m not aware of in JavaScript.

Questions:
Answers:

Well, here is my answer — mostly because I was not satisfied with the references in the other (correct) answers — expressions for property names in [ ] are always coereced to strings and this behavior is well defined in the specification. Thus, depending upon interpretation of the quote in question, it can be taken as misleading and/or incorrect.

However, the quote does not presume that x[42] and x["42"] are different; it states — with the misleading exclusion of other primitives and details — that only strings and numbers are usable as “hash keys” (really property names) under normal property resolution and, in this sense, the quote is arguably correct.

These rules are from Standard ECMA-262 ECMAScript Language Specification 5th edition (December 2009)

From section “11.2.1 Property Accessors” (production rules omitted):

The production MemberExpression : MemberExpression [ Expression ] is evaluated as follows:

  1. Let baseReference be the result of evaluating MemberExpression.
  2. Let baseValue be GetValue(baseReference).
  3. Let propertyNameReference be the result of evaluating Expression.
  4. Let propertyNameValue be GetValue(propertyNameReference).
  5. Call CheckObjectCoercible(baseValue).
  6. Let propertyNameString be ToString(propertyNameValue).
  7. If the syntactic production that is being evaluated is contained in strict mode code, let strict be true, else let
    strict be false.
  8. Return a value of type Reference whose base value is baseValue and whose referenced name is
    propertyNameString, and whose strict mode flag is strict.

Happy coding.

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Answers:

Here is a functor Array. It is useful when using a Functional Programming paradigm.

javascript:
    alert(["Using  ",window.navigator.userAgent] ); 
    FunctorRA=[]; f=function(){return f}; g=function(x){return x};
    FunctorRA[f]=43;
    FunctorRA[g(function(){})]="generic";
    FunctorRA[g(g)]="idempotent";
    alert(FunctorRA);
    for (i in FunctorRA)
        alert([ "Functor[ ", i,
                "]'s\n\n value is \n\n", FunctorRA[i]].join(""));

displays:

Using  ,Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.3) Gecko/20100423
    Ubuntu/10.04 (lucid) Firefox/3.6.3

an empty alert and then:

Functor[ function () {
    return f;
}]'s

 value is 

43

etc.

Note Bene:

  • alert( FunctorRA ) shows .toString() does not enumerate non-numeric indexes
  • FunctorRA is a generic object in array’s “clothing”
  • there is no direct . syntactic equivalent ( even with string eval coercion )
    see Dynamic function name in javascript? for details on how :, a colon, can be embedded in a function name even though it is usually a delimiter to syntactically delineate initializer properties, labels, ?: (conditional expressions) etc. An analogous problem exists with . requiring the escaping of all syntactically significant JavaScript character codes such as (){}\n ., … . The [] construct effectively does this for the parenthetically contained expression, as a whole, presumably by deferring the evaluation of the string to be an object of type String. This is similar to the fact 43.x=2 is verboten but not if 43 is represented as an object of type Number by (43).x=2 or 43["x"]=2.
    (proof: javascript:alert( [ (43).x=2, 43["x"]=2 ] ) displays 2,2 but javascript:alert(43.x=2) generates an error).
Questions:
Answers:

Yes, keys can be numbers. In fact, the specification utilizes the same generic mapping functions for both Objects and Arrays.

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Answers:

Well everything that you use for the key is converted to string because of hashing function that hashes strings to a nearly unique short set of bytes that can be interpreted as an integer, cause integer comparison during the linear search is low level and fast.(JavaScript objects are hash tables). You stringify objects and arrays to use them as a key. Keys can be unlimited in size. But beware, cause javascript objects are unordered.

Yesterday and today, I’ve tried to wrap my head around the problems appearing in this domain and I’ve written these solutions.
First one is a custom implementation of a hash table, http://stamat.wordpress.com/javascript-quickly-find-very-large-objects-in-a-large-array/
And there everything is explained in laymans terms…

other one is an upgrade to the first one, and it is a deterministic JSON stringify, that stringifies objects with alphabetically ordered properties: http://stamat.wordpress.com/2013/07/03/javascript-object-ordered-property-stringify/

Check it out 🙂

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