Converting one Enum to another in JavaScript

Question

EDIT

As comment suggests, Enum is not part of JavaScript but part of TypeScript. I intentionally left original title as one may make mistake like me.

I have two enums with the same keys but different values.

enum RowStates {
    editing = 0,
    sentToApproval,
    approved
    // ...
}

enum RowColors {
    editing = '#ffffff',
    sentToApproval = '#ffffcc',
    approved = '#ccffb3'
    // ...
}

And I have some function to do convertion:

function Convert (rowState) {
// What should be here to return rowColor?
// Using switch (rowState) is obvious, but may be other solution exist?
}

emun is not a javascript keyword.

Maheer Ali
– Maheer Ali

2019-03-18 03:48:58 +00:00
Commented Mar 18, 2019 at 3:48 — Maheer Ali
– Maheer Ali, Commented Mar 18, 2019 at 3:48

Robby Cornelissen · Accepted Answer · 2021-07-27 00:32:46Z

16

TypeScript enums allow you to do a reverse mapping:

enum RowStates {
    editing = 0,
    sentToApproval,
    approved
}

enum RowColors {
    editing = '#ffffff',
    sentToApproval = '#ffffcc',
    approved = '#ccffb3'
}

function convert(rowState: RowStates) {
    return RowColors[RowStates[rowState] as keyof typeof RowColors];
}

console.log(convert(RowStates.sentToApproval)); // prints '#ffffcc'

edited Jul 27, 2021 at 0:32

answered Mar 18, 2019 at 3:53

Robby Cornelissen

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3 Comments

Peters_ Over a year ago

I have two enums with exactly same values, only names differ. I need to convert one to another, but the solution above does not work. "Property 'X' does not exist on type 'typeof XYZEnum'. Do you have any idea why?

Robby Cornelissen Over a year ago

@Peters No idea. I've just tested the solution above and (with a minor modification), it still works in the current TypeScript version (4.3.5). I think your best bet is to ask a new question and explain what's different about your specific case.

Peters_ Over a year ago

I noticed that the difference is that for me "rowState" can be undefined, so it is declared with "?" at the end. Both enums are generated from java code and in typescript look like this: enum MyEnum {AB= 'A_B', CD= 'C_D'}. The error message is "TS2551 Property 'A_B' does not exist. Did you mean 'AB'?"

Kamil Kiełczewski · Accepted Answer · 2021-12-02 10:25:34Z

4

try

function Convert (rowState: RowStates): RowColors {
    return RowColors[RowStates[rowState]];
}

working example here

Update

As poloapolo notice in his comment in current TS 4.5.2 version (and probably in some earlier versions too) this solution produce linter error

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'typeof RowColors

it can be avoid as follows (this is alternative to Robby answer)

function Convert (rowState: RowStates): RowColors {
  return new Map(Object.entries(RowColors)).get(RowStates[rowState]) as RowColors
}

In both solutions we can add some kind of validation in case when there is key in RowStates which not exists in RowColors

edited Dec 2, 2021 at 10:25

answered Mar 18, 2019 at 4:01

Kamil Kiełczewski

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2 Comments

poloapolo Over a year ago

Accessing RowColors without casting RowState item to keyof typeof RowColors, produces this linter error: "Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'typeof RowColors'"

Kamil Kiełczewski Over a year ago

@poloapolo I update answer

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