What is the difference between a key and an identifier in JS?

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Regarding objects, why do they have keys that are associated with their data. For instance: a variable with the identifier 'num' will hold a reference to that value in memory, but why do objects not use this principle? Instead they have keys that are, correct me if I am wrong, actual values themselves that are associated with a property of an object. I hope I am explaining this correctly. In short, why do objects not use identifers like standard variables and instead have keys associated with them that are their own values. I checked it and JS says they are of type 'string'. Standard variables have an identifier that is not a value but a reference to said value, why do objects have this additional layer of complexity?

Why do objects behave like this-

Key variable associated with variable

And not like this (like normal declaration with let, var, const)-

Identifier references variable

Note: I am new to JS

javascript
1 Answers

A key is a runtime value whereas an identifier is a syntactical element.

Objects in JS do not have a fixed shape, you can create, update and delete properties in them at will. And you can do so dynamically, without knowing the names of the properties when you write the program, using bracket notation: obj[prop] = val. And similarly you can loop over the keys of an object with for (const prop in obj). This is why keys are strings (or symbols) - you need to access them with/as string values.

The term "identifier" refers to specific syntax in the grammar of the language. It is written by the programmer and parsed by the compiler. Notice that in the expression obj.prop, both "obj" and "prop" are identifiers, so they don't even have to refer to variables. They are just the subset of strings you can write to name variables and properties with dot syntax. They are not values you can access from a javascript program, and don't really exist at runtime any longer.

You might have meant variables though. Variables are distinguished by identifiers, the same identifier in the same scope will refer to the same variable. This distinction happens by name, treating the variable names as strings in the compiler. You just never get in contact with those strings from JS code. When the code is executed, for each declared name a variable with a value will be created in an environment structure for that scope.

So there is some similarity:

  • An object consists of property keys associated with property values.
  • A scope environment consists of variable names associated with variable values.

And actually, this similarity is (or has been) exploited by usage of eval and with statements and the global object, that actually allow the dynamic creation and assignment of variables by name. For these, the interpreter has to keep the variable names (as strings) in memory. It's inefficient and despised though. When they (both object keys and variable names) are not dynamic, the interpreter will be able to optimise them a lot.

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