Behaviour of undefined and null keys in ecmascript 5 or 6

Question

I'm interested in knowing what is the correct behaviour when object is accessed by undefined or null key in javascript.

I checked its behaviour in Chrome 46, Firefox 42 and node 0.12.0 and it's the same:

var obj = {};
obj[undefined] = 'foo';
obj[undefined]; // 'foo'
obj['undefined']; // 'foo'
obj; // {undefined: 'foo'}

or:

var obj = {};
obj[null] = 'foo';
obj[null]; // 'foo'
obj['null']; // 'foo'
obj; // {null: 'foo'}

Then, it seems that undefined and null are coerced to strings 'undefined' and 'null'. However if I read ecmascript specs and I see that RequireObjectCoercible is used. Reading RequireObjectCoercible specs, I see that when it is passed undefined or null as argument, it throws a TypeError exception.

Therefore I don't understand why I reproduce another behaviour when I check it.

Edit:

Thanks for Andreas and Bergi

I understand that the behaviour is this because:

12.3.2.1 Runtime Semantics: Evaluation

[...]

Let propertyKey be ToPropertyKey(propertyNameValue).

7.1.14 ToPropertyKey ( argument )

[...]

Return ToString(key).

7.1.12 ToString ( argument )

The abstract operation ToString converts argument to a value of type String according to Table 12:

[...]

Undefined | Return "undefined".

Null | Return "null".

Right?

In obj[undefined], obj['undefined'], obj[null] the part in [ ] equals the [ Expression ] part in MemberExpression [ Expression ]. Therefor you have to have a look at 4., 5., 6., 9. and 10. — Andreas
– Andreas, Commented Dec 19, 2015 at 12:10

Bergi · Accepted Answer · 2015-12-19 12:26:57Z

1

RequireObjectCoercible is called on the base value, not on the property, before the property reference is constructed. And obj is coercible to an object :-) It does throw when you are doing stuff like undefined.x or null[y].

answered Dec 19, 2015 at 12:26

Bergi

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1 Comment

MaximPro Over a year ago

One moment! When is the RequireObjectCoercible function called with an argument when ToObject is called? How it's works if I calling it so: RequireObjectCoercible(7)?

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