JavaScript property access using brackets - javascript

If I declare the following in my Chrome console:
var object = {0:0, 1:1}
I can call object[0] and object[1] and get their values. I can also call object["0"] and object["1"]. Next, if I declare:
var object = {"0":0, "1":1}
I can also make all four of the above calls. But if I declare:
var object = {a:0, 1:1}
I get a ReferenceError of "a is not defined" when I call object[a], but object["a"] returns 0, even though the property name in the declaration is not a string. I guess JavaScript thinks I'm calling a variable that doesn't exist in the first example. But why do calling object[0] and object["0"] both work? It seems that JavaScript is doing some kind of automatic conversion for numbers (presumably since they can't be variable names), but what are the rules for this? And is this behavior universal to other places it might come up or just to the bracket notation for objects?

When you use brackets, the expression inside the brackets is evaluated. What's the value of the expression
a
?? Well, if "a" isn't a declared variable, it's nonsense. When you use . notation, the identifier (and it must be an identifier) following the operator is treated as a string. It's just the way the language works.

The reason you're getting a ReferenceError for object[a] is because a literal a is a variable in javascript. "a" is a string containing the letter a.
You can use the dot notation object.a or the bracket notation with object["a"]
object.a; //=> 0
object["a"]; //=> 0
object[1]; //=> 1
object["1"]; //=> 1
Or you can use a variable for access
var x = "a";
object[x]; //=> 0
var y = 1;
object[y]; //=> 1

You are correct.
a there is a token which the engine assumes is a variable.
If you type "a" JS knows it's a string-primitive.
If you type 0, JS knows it's a number-primitive.
So on top of obj.a, obj["a"], obj[0], obj["0"], you can also say:
var a = 0;
obj[a]; // 0
Your app is exploding, because a hasn't been defined yet, and now you want to use it.
And yes, this is the expected behaviour.
What's inside of the brackets isn't seen as a "part" of the object -- it's a way of saying "give me the value of the object which is referenced by this key", where the key might be a number or string (or something that can be coerced into a string or number).
In the future, with maps and weakmaps, you would actually be able to use other objects/functions as keys as well.
var obj = new Map(),
func = function () { },
el = document.getElementById("myId");
obj[func] = 1;
obj[el] = 2;
Right now, these things technically work... ...but only because they're converted to their string values... ...so if you had two functions which were written the same (but technically two different objects), you would overwrite values, currently.
Inside of a map, they'd be treated as separate objects.
Using DOM elements is even worse, right now, as it might be useful to store hundreds of those and bind references to them, so that you don't have to keep looking for them... ...but for now, you need to make a unique ID number/key for each one, and store that, and remember the keys, and then create a child object to hold the data you want...
Whereas in the future, with maps, you could say:
var my_els = document.querySelector(".lots-of-els");
for (let el of my_els /* also the future */) {
console.log( my_map_of_data[el].info );
}

Related

Why does JavaScript convert an array of one string to a string, when used as an object key? [duplicate]

This question already has answers here:
Why can I access object property with an array?
(2 answers)
Closed 2 years ago.
I ran into a scenario where JavaScript behaves in a way that is somewhat baffling to me.
Let's say we have an object with two keys foo & bar.
a = { foo: 1, bar: 2 }
Then, I have an array of strings, in this case one 'foo'
b = ['foo']
I would expect the following:
a[b] == undefined
a[b[0]] == 1
BUT, this is what happens:
a[b] == 1
a[b[0]] == 1
Why does JavaScript convert ['foo'] -> 'foo' when used as a key?
Does anyone out there know the reason?
How can this be prevented?
let a = { foo: 1, bar: 2 }
let b = ['foo']
console.log(a[b] == 1) // expected a[b] to be undefined
console.log(a[b[0]] == 1) // expected a[b] to be 1
All the object keys are string, so it eventually convert everything you place inside [] (Bracket notation) to string, if it's an expression it evaluates the expression and convert it's value to string and use as key
console.log(['foo'].toString())
Have a look at this example to understand, here [a] eventually converts a toString using a.toString() and then set it as key to b object
let a = { a : 1}
let b = {
[a] : a
}
// object converted to string
console.log(a.toString())
// object built using [] computed property access
console.log(b)
How can i stop this
In practical scenarios you should never do this, but just to illustrate, you can intercept or override the toString method of your object and return value as string with [] around:
let a = { foo: 1, bar: 2 }
let b = ['foo']
b.toString = function() {
let string = this.join(',')
return "[" + string + "]"
}
console.log(b.toString())
console.log(a[b])
When using an array as a key, javascript call the 'toString()' method of that array, and then try to find the stringified version of the array as the key. And if you call ['foo'].toString() you see this method returns "foo".
Why does JavaScript convert ['foo'] -> 'foo' when used as a key?
Does anyone out there know the reason?
Any time there is confusion as to why JavaScript acts in a way which may be unexpected, then looking at the language definition is the surefire way to exactly figure out what happened.
https://www.ecma-international.org/ecma-262/10.0/ is the most current language definition at the time of posting this.
First, you will want to find the area pertaining to Array access. It is in language lingo though.
12.3.2.1 Runtime Semantics: Evaluation
MemberExpression : MemberExpression [ Expression ]
...
3. Let propertyNameReference be the result of evaluating Expression.
4. Let propertyNameValue be ? GetValue(propertyNameReference).
6. Let propertyKey be ? ToPropertyKey(propertyNameValue).
So, what is happening here is you are accessing your array (the MemberExpression) using [] with an Expression.
In order to access with [] the Expression will be evaluated, and then GetValue will be called. Then ToPropertyKey will be called.
propertyNameReference = Evaluate Expression b = b
propertyNameValue = GetValue(propertyNameReference) = ['foo']
propertyKey = ToPropertyKey(propertyNameValue) = 'foo'
ToPropertyKey, in our situation, leads to ToPrimitive and then to ToOrdinaryPrimitive which states that we should call "toString" on the argument (['foo'] in our case).
This is where the implementation comes in to play. On the implementation side,
The Array object overrides the toString method of Object. For Array objects, the toString method joins the array and returns one string containing each array element separated by commas" MDN - Array toString
When there is only one value in the array, the result will simply be that value.
How can this be prevented?
This is the current way it is implemented. In order to change that, you must either change the default implementation, use detection to prevent the call, or use guidance to prevent the call.
Guidance
Document and enforce calling mechanisms in your code. This may not always be possible. It is at the very least reasonable to expect programmers to not call property access with arrays though.
Detection
This will depend on the current environment. With the most recent iteration of JavaScript, you can use type enforcement to ensure that property access is Number or String. Typescript makes this rather easy (here is a good example). It would essentially just require the access to be defined as:
function ArrayAccess(value: string | number) {
and this would prevent anyone from using the array as an accessor value.
Default Implementation
Changing the default implementation is a terrible idea. It will more than likely cause all sorts of breaking changes, and should not be done. However, just for completeness, here is what it would look like. Primarily I am showing this so you can hopefully recognize it if you see it somewhere and then kill it with fire (or check in some code to fix it if there were no spiders near it).
var arrayToString = [].toString;
Array.prototype.toString = function(){
if(this.length === 1) return;
return arrayToString.call(this);
};
Changing the instance implementation is not much of a better idea either. That is covered by #Code Maniac in a separate answer. "In practical scenarios you should never do this" #Code Maniac states, which I also agree with.
When using an array as a key, javascript call the 'toString()' method of that array, and then try to find the stringified version of the array as the key. And if you call ['foo'].toString() you see this method returns "foo".

Javascript: adding a key-value pair to array

I noticed that I can add a key value pair to an array (not an object, an array).
var a = []; // create the array
a[0] = "test"; // conventionally setting an index of 0 to a value
a["foo"] = "bar"; // this actually sets a "key" of the array to "bar"
If I try to get the value of a.foo or `a["foo"], I simply get "bar". No errors raised.
I know that a Javascript Array is actually an object, but with special rules, but it feels weird that this doesn't throw an error.
I'm using the latest version of Chrome.
Is there an actual use case where this is ok to do? What is common practice around this fact?
Now try this:
> Object.getOwnPropertyNames(a)
[ '0', 'length', 'foo' ]
> a.length
1
Welcome to JavaScript! It's such a wonderful place...
Yes, you can create named properties on an array. No, you probably shouldn't. They won't be counted in Array.length, and most developers who read your code will be experiencing a moment of confusion that is unlikely to lead to a positive outcome.
If you need a named property, use a (non-array) object.

javascript primitives vs object references

I've been using JavaScript for years and this one has me stumped. As I understood things, when defining a var, one of two things will happen:
If the expression is a primitive, var is defined as a new instance of that primitive with no reference to the passed expression.
If the expression is an object literal, a new object is created.
If the expression is an existing object, var will reference the object and any future changes to the object will be reflected.
However, I've run into a situation where case 3 doesn't apply:
var obj = {body: {'a': 1, 'b': 2, 'c': 3}};
var ref = obj.body;
ref = JSON.stringify(ref);
console.log(typeof ref); // string
console.log(typeof obj.body); // object
Since ref is defined as the body property of obj, I thought redefining ref as a string also would affect obj.body. So what am I missing?
JSON.stringify is a method which takes an object and returns its string representation, it doesn't change anything. By doing ref = x you make ref point to another thing, it doesn't affect what was there before assignment.
That simply means, you are no more referencing obj.body.body and referencing to something else.
var ref = obj.body;
//ref holding obj.body now any changes to ref will effect obj.body.
ref = JSON.stringify(ref);
//ref holding a String returned by `stringify()` now any changes to ref will effect obj.body.
You see ?? You just changing the ref with different values. Not really changing anything on obj
Primitives are immutable. If there’s a difference in how they would behave compared to objects, you can’t observe that, so forget all that stuff about copying. Let’s talk instead in terms of “things”! Objects and primitives are both things. When you assign a thing to a variable, you are not copying the thing.
var x = literally any value;
var y = x;
x and y are both variables that contain the same thing. If you change the thing, it doesn’t matter where you access it from in the future; the thing changed. If you change the thing the variable contains, the thing it contained before is not affected.
var z = some other value;
y = z; // y now contains the same thing as z instead of the same thing as x
// only variables changed, and the things did not
There are a lot of answers that talk about this in other terms but I enjoy technical language.
tl;dr: For all intents and purposes, the distinction between objects and primitives in JavaScript is not a useful one.
ts;iwrse: This article about Python applies to JavaScript just as much.

Can an Object object be coerced into an Array object?

Crockford writes in http://javascript.crockford.com/survey.html:
"There are two ways to make a new array:
var myArray = [];
var myArray = new Array();"
So I'm confused by these two lines in some AJAX code I am reading:
var obj={}; // obj is an Object object (i.e. a hash table)
obj[4] = 'x'; // now obj is suddenly an Array object via an integer key?
In JavaScript are an object and an array really just the same thing, but with a variant on the key type?
In other words, is this the same as in php where we can use either a name (string) or an integer for a hash key?
I've Googled for an answer on this but can't seem to nail down an article which discusses this issue.
One possibility that comes to mind is that perhaps the first line is syntactic lint because the 2nd line overwrites the previous definition of obj as it creates a new Array object.
it does not become an array, it is simply an Object with a '4' property, like this:
var obj = {
'4': 'x'
};
it is just converted to a string when used as a property like obj['4'] = 'x';
Everything but primitive datatypes is an object in JavaScript. Objects can have a properties and there are two ways to access object properties:
Dot notation, foo.bar, which you can use as long as the property name is a valid identifier.
Bracket notation, foo['bar'] which you have to use if the key is not a valid identifier [spec]. For example, if it is a number, or contains a space or you have a variable with the name.
Hence, bracket notation is not a characteristic of arrays and if you see it, it does not mean the value is an array. It is simple one of two ways of accessing properties.
The elements of an array are just properties with numeric keys. Arrays are built on top of objects and implement some additional methods which treat these numeric properties in a special way. For example the .length property is automatically updated when you add new elements. But ultimately they are just normal properties.
In your example you have a simple object. You have to access the property with obj[4] or obj['4'] because obj.4 is invalid since 4 is not a valid identifier (basically everything that you can use as variable name is a valid identifier. var 4 = 'foo'; is invalid).
And since arrays are just objects, if you could use numbers as identifiers, you were also able to access an element with arr.4.
As far as I know, no, an object can't be coerced into an array. But, it can look and act like an array, and that's what's happening here. Numbers, and anything else that can be coerced to a string, are perfectly valid property names for Javascript objects, so
obj[4] = 1;
obj['spam'] = 2;
are both valid ways of setting a property on the object. That doesn't make the object an array. An Array is a special class of object with specific methods (.slice(), .concat(), etc) and a length property that's kept up to date with the number of items in the array.
Yes
Javascript Array is very different from tradition array, you can think of it as object.
var array = [1,2,3] is equivalent to var object = {'0' : 1, '1' : 2, '2' : 3}
except array inherited from Array.prototype and object inherited from Object.prototype, where Array.prototype will contain method such as length.
Javascript is a loosely-typed, prototype-based language. Even primitive types like a boolean can be treated like an object (though you aren't going to get far). Almost everything in javascript is, at root, an object.
Understanding this, an array IS an object. You can arbitrarily add properties to any object:
var xml = new XMLHttpRequest();
xml[4] = 'x';
console.log(xml);
That object is still an instance of XMLHttpRequest. It now has a property labeled 4 with a value of x. You can treat anything like this -- even a function:
var test_func = function () {
alert('woah!');
}
test_func[4] = 'x';
console.log(test_func[4]);
The take-away here is that the obj[key] = value notation is NOT indicative of an "array" type, like it is in languages such as PHP. Rather, it is an alternate way to access properties of any object, and is equivalent to obj.key = value (you can't use obj.4 = 'x', though, that's invalid syntax). The other take-away is that any object in javascript can be modified or used in pretty much any way. You shouldn't misuse objects, but you can
Check it out here: http://jsfiddle.net/w2AqJ/
Documentation
Array on MDN - https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array
Javascript "associative arrays" considered harmful by Andrew Dupont - http://andrewdupont.net/2006/05/18/javascript-associative-arrays-considered-harmful/

JavaScript: Use Object As Array

Are there any pitfalls to code like this?
var Foo = function() {
this.bar = function() { return 'bar'; };
};
var f = new Foo();
f[0] = 'hi';
f[1] = 'there';
Note that I'm creating a new function object with some misc properties, and then I'm treating the object like an array. Also how are the array values being stored in the object? Are 0 and 1 treated like property names?
Well, yes, 0, and 1 will be just two property names.
When you assign a property with the bracket notation, the expression between the brackets will be converted to String, and that string will be used as the property name.
In fact, even the indexes for real arrays are just that, properties:
var realArray = ['a'];
realArray.hasOwnProperty('0'); // true
The difference is that real array objects on every property assignment that correspond to a valid index[1], track internally the value of their length property.
That's one of the reasons why "subclassing" array objects is difficult, even with the new ECMAScript 5 extensions, or at the moment also with the proposed ECMAScript-Harmony Proxies 2 3, can't be completely done, in a stanard way.
That can be a problem, depending on how you plan to iterate the numeric properties.
If you enumerate the properties with the for-in statement, other members will appear, not just the numeric properties.
[1] A valid array index is any unsigned 32-bit integer in the range of 0 to (2^32)-1.
I think you might have problems if you try to loop through that with a for in loop; the loop will also get bar. There are ways around this, so you just have to be careful.
If you want to extend Array behavious please use : Array.prototype.yourfunc = function()....

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