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@dgcoffman
Last active March 12, 2017 03:03
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Description of how JavaScript objects work
Objects have four states:
properties can be added | deleted | modified* | do it with | check with
Default √ | √ | √ | n/a | n/a
preventExtensions X | √ | √ | preventExtensions | isExtensible
seal X | X | √ | seal | isSealed
freeze X | X | X | freeze | isFrozen
* by changing any of the four attributes below
You can make an object in three different ways:
new Object(), Object.create(), or using the literal notation (initializer notation)
Properties come in two flavors:
I. Data properties
II. Accessor properties
Data properties have four attributes:
1. value
The property's value.
2. writable
true or false. Whether the property's value can be written to. (For example, o["p"] = 1)
3. enumerable
true or false. Whether the property shows in some loop constructs, such as for (var x in o) {…} and Object.keys(o).
4. configurable
true or false. Whether the property can be deleted (For example, delete o["p"]) and whether its attributes can be changed.
Accessor properties have the same writeable and configurable attributes as data properties, but in addition (respectively):
5. get
6. set
Properties an be created with:
Object.defineProperty(obj, 'propName', { value, writable, enumerable, configurable });
Defaults given an empty object are:
{ value: undefined, writable: false, enumerable: false, configurable: false }
However, creating a new property by assigning to it uses defaults:
{ value: "your value", writable: true, enumerable: true, configurable: true }
Property attributes can be accessed with:
Object.getOwnPropertyDescriptor(obj) or
Object.getOwnPropertyDescriptor(obj, "propName") or
Reflect.getOwnPropertyDescriptor(obj, "propName")
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