# Enums

# How to get all enum values

enum SomeEnum { A, B }

let enumValues:Array<string>= [];

for(let value in SomeEnum) {
    if(typeof SomeEnum[value] === 'number') {
        enumValues.push(value);
    }
}

enumValues.forEach(v=> console.log(v))
//A
//B

# Enums with explicit values

By default all enum values are resolved to numbers. Let's say if you have something like

enum MimeType {
  JPEG,
  PNG,
  PDF
}

the real value behind e.g. MimeType.PDF will be 2.

But some of the time it is important to have the enum resolve to a different type. E.g. you receive the value from backend / frontend / another system which is definitely a string. This could be a pain, but luckily there is this method:

enum MimeType {
  JPEG = <any>'image/jpeg',
  PNG = <any>'image/png',
  PDF = <any>'application/pdf'
}

This resolves the MimeType.PDF to application/pdf.

Since TypeScript 2.4 it's possible to declare string enums (opens new window):

enum MimeType {
  JPEG = 'image/jpeg',
  PNG = 'image/png',
  PDF = 'application/pdf',
}

You can explicitly provide numeric values using the same method

enum MyType {
   Value = 3,
   ValueEx = 30,
   ValueEx2 = 300
}

Fancier types also work, since non-const enums are real objects at runtime, for example

enum FancyType {
   OneArr = <any>[1],
   TwoArr = <any>[2, 2],
   ThreeArr = <any>[3, 3, 3]
}

becomes

var FancyType;
(function (FancyType) {
    FancyType[FancyType["OneArr"] = [1]] = "OneArr";
    FancyType[FancyType["TwoArr"] = [2, 2]] = "TwoArr";
    FancyType[FancyType["ThreeArr"] = [3, 3, 3]] = "ThreeArr";
})(FancyType || (FancyType = {}));

# Extending enums without custom enum implementation

enum SourceEnum {
  value1 = <any>'value1',
  value2 = <any>'value2'
}

enum AdditionToSourceEnum {
  value3 = <any>'value3',
  value4 = <any>'value4'
}

// we need this type for TypeScript to resolve the types correctly
type TestEnumType = SourceEnum | AdditionToSourceEnum;
// and we need this value "instance" to use values
let TestEnum = Object.assign({}, SourceEnum, AdditionToSourceEnum);
// also works fine the TypeScript 2 feature
// let TestEnum = { ...SourceEnum, ...AdditionToSourceEnum };

function check(test: TestEnumType) {
  return test === TestEnum.value2;
}

console.log(TestEnum.value1);
console.log(TestEnum.value2 === <any>'value2');
console.log(check(TestEnum.value2));
console.log(check(TestEnum.value3));

# Custom enum implementation: extends for enums

Sometimes it is required to implement Enum on your own. E.g. there is no clear way to extend other enums. Custom implementation allows this:

class Enum {
  constructor(protected value: string) {}

  public toString() {
    return String(this.value);
  }

  public is(value: Enum | string) {
    return this.value = value.toString();
  }
}

class SourceEnum extends Enum {
  public static value1 = new SourceEnum('value1');
  public static value2 = new SourceEnum('value2');
}

class TestEnum extends SourceEnum {
  public static value3 = new TestEnum('value3');
  public static value4 = new TestEnum('value4');
}

function check(test: TestEnum) {
  return test === TestEnum.value2;
}

let value1 = TestEnum.value1;

console.log(value1 + 'hello');
console.log(value1.toString() === 'value1');
console.log(value1.is('value1'));
console.log(!TestEnum.value3.is(TestEnum.value3));
console.log(check(TestEnum.value2));
// this works but perhaps your TSLint would complain
// attention! does not work with ===
// use .is() instead
console.log(TestEnum.value1 == <any>'value1');

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