enums showing double the amount of entries; "extra" key value pairs are the flipped versions of the original key-value pairs

[kptevpro]

Bug Report

🔎 Search Terms

is:issue enum duplicate key value double extra entries flipped

🕗 Version & Regression Information

Found in v~4.7.2

⏯ Playground Link

Playground link with relevant code

💻 Code

enum TransactionTypes {
    A = 1,
    B = 2,
    C = 3,
    D = 4,
    E = 5,
    F = 6,
    G = 7,
    H = 8,
}

console.log('Keys:' + Object.keys(TransactionTypes))
console.log('Values: ' + Object.values(TransactionTypes))

for(let i = 0; i < Object.keys(TransactionTypes).length; i++) {
    console.log(`Pair: ${Object.keys(TransactionTypes)[i]} - ${Object.values(TransactionTypes)[i]}`) 
}

🙁 Actual behavior

Enums shows double the amount of entries; shows key-value pairs twice where the second set (extra entries) are the original keys and values 'flipped'.

🙂 Expected behavior

Show key value pairs once; only one set of the key values declared.

[MartinJohns]

This is working as intended. Check the generated JavaScript. See also #51150.

[fatcerberus]

For the record the reason for this is so you can evaluate TransactionTypes[x] in the debugger to find out which enum member x (just a number at runtime) refers to.

[RyanCavanaugh]

It would be quite the interesting bug for a 100% accurate reverse map to appear in the output (and go unnoticed for ten years). But no, this is there on purpose.

[aitoraznar]

Hi, it happened to me when using UPPERCASE keys in the enum.

// Duplicated keys in uppercase && lowercase
enum TransactionTypes {
A = 1,
B = 2,
C = 3,
}

// Correct keys
enum TransactionTypes {
a = 1,
b = 2,
c = 3,
}

[RyanCavanaugh]

Please log a new issue with concrete repro steps if you think you've encountered a bug.