Determine the number of enum elements TypeScript

If you have an enum with numbers that counts from 0, you can insert a placeholder for the size as last element, like so:

enum ERROR {
  UNKNOWN = 0,
  INVALID_TYPE,
  BAD_ADDRESS,
  ...,
  __LENGTH
}

Then using ERROR.__LENGTH will be the size of the enum, no matter how many elements you insert.

For an arbitrary offset, you can keep track of it:

enum ERROR {
  __START = 7,
  INVALID_TYPE,
  BAD_ADDRESS,
  __LENGTH
}

in which case the amount of meaningful errors would be ERROR.__LENGTH - (ERROR.__START + 1)

Typescript does not provide a standard method to get the number of enum elements. But given the the implementation of enum reverse mapping, the following works:

Object.keys(ExampleEnum).length / 2;

Note, for string enums you do not divide by two as no reverse mapping properties are generated:

Object.keys(StringEnum).length;

Enums with mixed strings and numeric values can be calculated by:

Object.keys(ExampleEnum).filter(isNaN).length;

The accepted answer doesn't work in any enum versions that have a value attached to them. It only works with the most basic enums. Here is my version that works in all types:

export function enumElementCount(enumName: any): number {
    let count = 0
    for(let item in enumName) {
        if(isNaN(Number(item))) count++
    }
    return count
}

Here is test code for mocha:

it('enumElementCounts', function() {
    enum IntEnum { one, two }
    enum StringEnum { one = 'oneman', two = 'twoman', three = 'threeman' }
    enum NumberEnum { lol = 3, mom = 4, kok = 5, pop = 6 }
    expect(enumElementCount(IntEnum)).to.equal(2)
    expect(enumElementCount(StringEnum)).to.equal(3)
    expect(enumElementCount(NumberEnum)).to.equal(4)
})