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First, what is it?

An enumeration is a collection of named integer constants used to declare a set of named constants, which can be defined as an enumeration type when a variable has several possible values

In popular terms, an enumeration is a collection of all possible values of an object

It is also common in daily life. For example, SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY and SATURDAY indicating the day of the week can be regarded as an enumeration.

The description of enumerations is similar to structure and federation in the form of:

Enum enumeration name {

Identifier ① [= integer constant]

Identifier ② [= integer constant]

...

Identifier N [= integer constant]

} enumerate variables

2. Use

The use of enumerations is defined by the enum keyword, as follows:

Enum xxx {...}

The way to declare keywords as enumerated types is as follows:

/ / declare d to be an enumerated type Directionlet d: Direction

Types can be divided into:

Numeric enumeration

String enumeration

Heterogeneous enumeration

Numeric enumeration

When we declare that an enumerated type is, although no values are assigned to them, their values are actually the default numeric types, and the default accumulates from 0:

Enum Direction {Up, / / defaults to 0 Down, / / defaults to 1 Left, / / defaults to 2 Right / / defaults to 3} console.log (Direction.Up = 0); / / trueconsole.log (Direction.Down = 1); / / trueconsole.log (Direction.Left = 2); / / trueconsole.log (Direction.Right = 3); / / true

If we assign the first value, the subsequent values will also accumulate 1 based on the previous value:

Enum Direction {Up = 10, Down, Left, Right} console.log (Direction.Up, Direction.Down, Direction.Left, Direction.Right); / / 10 11 12 13 string enumeration

The value of an enumeration type can actually be a string type:

Enum Direction {Up = 'Up', Down =' Down', Left = 'Left', Right =' Right'} console.log (Direction ['Right'], Direction.Up); / / Right Up

If a variable is set as a string, the subsequent fields also need to be assigned a string, otherwise an error is reported:

Enum Direction {Up = 'UP', Down, / / error TS1061: Enum member must have initializer Left, / / error TS1061: Enum member must have initializer Right / / error TS1061: Enum member must have initializer} heterogeneous enumeration

Combine numeric enumeration with string enumeration, as follows:

Enum BooleanLikeHeterogeneousEnum {No = 0, Yes = "YES",}

In general, we rarely use heterogeneous enumerations.

Essence

Now an example of an enumeration is as follows:

Enum Direction {Up, Down, Left, Right}

After compilation, the javascript is as follows:

Var Direction; (function (Direction) {Direction [Direction ["Up"] = 0] = "Up"; Direction [Direction ["Down"] = 1] = "Down"; Direction [Direction ["Left"] = 2] = "Left"; Direction [Direction ["Right"] = 3] = "Right";}) (Direction | | (Direction = {}))

As you can see in the above code, Direction [Direction ["Up"] = 0] = "Up" can be divided into

Direction ["Up"] = 0

Direction [0] = "Up"

So after defining the enumeration type, you can get the corresponding value through positive and negative mapping, as follows:

Enum Direction {Up, Down, Left, Right} console.log (Direction.Up = 0); / / trueconsole.log (Direction [0]); / / Up

And multiple defined enumerations can be merged, as follows:

Enum Direction {Up = 'Up', Down =' Down', Left = 'Left', Right =' Right'} enum Direction {Center = 1}

After compilation, the js code is as follows:

Var Direction; (function (Direction) {Direction ["Up"] = "Up"; Direction ["Down"] = "Down"; Direction ["Left"] = "Left"; Direction ["Right"] = "Right";}) (Direction | (Direction = {})); (function (Direction) {Direction [Direction ["Center"] = 1] = "Center";}) (Direction | | (Direction = {}))

As you can see, the Direction object properties are overlaid

Third, application scenarios

Take back the example of life. The fields returned at the backend are marked with a date of 0-6, and enumerations can be used to improve the readability of the code, as shown below:

Enum Days {Sun, Mon, Tue, Wed, Thu, Fri, Sat}; console.log (Days ["Sun"] = 0); / / trueconsole.log (Days ["Mon"] = 1); / / trueconsole.log (Days ["Tue"] = = 2); / / trueconsole.log (Days ["Sat"] = 6); / / true

Including when the backend returns 0, 1, and so on, we can define it through enumeration, which can improve the readability of the code and facilitate subsequent maintenance.

Thank you for reading! This is the end of this article on "sample analysis of enumerated types in TypeScript". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it for more people to see!

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