How do I enumerate thee? Let me count the ways.

29 November 2023
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JavaScript

Why only string-named own enumerable data properties? JavaScript has a tremendous number of different constructs for enumerating the properties of an object, with different semantics of what subset they choose to enumerate:
— @erights

I have largely duplicated Mark Miller’s table for my own reference, and now yours:

Operation	P	NE	STR	SYM	output
`for (... in ...)`	P		STR		k*
`Object.keys`			STR		[k*]
`Object.values`			STR		[v*]
`Object.entries`			STR		[[k,v]*]
`{...obj}`			STR	SYM	{k:v*}
`Object.assign`			STR	SYM	{k:v*}
`Reflect.ownKeys`		NE	STR	SYM	[k*]
`Object.getOwnPropertyNames`		NE	STR		[k*]
`Object.getOwnPropertySymbols`		NE		SYM	[k*]
`Object.getOwnPropertyDescriptors`		NE	STR	SYM	{k:d*}

Legend:

P: Includes properties inherited from up the prototype chain. For example, {__proto__: {a: 10}, b: 20} has properties "a" and "b".
NE: Includes properties that are non-enumerable, like the length of an array or methods on classes.
STR: Includes properties named by a string.
SYM: Includes properties named by a symbol. For example, {[Symbol.toStringTag]: 'inline'} has a symbol property.
k* indicates keys.
[k*] indicates an array of keys.
[v*] indicates an array of values.
{k:v*} indicates an object mapping keys to values.
{k:d*} indicates an object mapping keys to property descriptors.

For objects with accessor properties (getters), JavaScript has two modes of copying, copy-by-value and copy-by-descriptor.

Output that contains a v indicates that the operation copied the value. The operation will synchronously capture a snapshot of the current value by calling the getter. Object.values, Object.entries, {...object}, and Objeect.assign call getter functions and copy-by-value.
When the output contains a d, the getter and setter are in the descriptor, as in Object.getOwnPropertyDescriptors. The operation does not call the getter and merely copies the descriptor.

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