CS Student at the University of Cincinnati

Passionate about building things that people love to use and exploring new technologies. A proud part of the hacker culture and communities.

Full stack web. Node.js. JavaScript. C++. Python. HTML5.

TypeScript Enum Iteration

Recently I ran into the issue with enums in TypeScript. Iterating over an enum is possible, but it spits out possibly unintended items. Here the for-in, not for-of, loop logs each item in the enum, numbers and strings:

enum Suit { Heart, Diamond, Spade, Club }

for (const suit in Suit) {
    console.log(suit);
}

// 0
// 1
// 2
// 3
// Heart
// Diamond
// Spade
// Club

So the solution seems straightforward… check if an item is a string. Now we should get no numbers:

for (const suit in Suit) {
    if (typeof suit === 'string') {
        console.log(suit);
    }
}

// 0
// 1
// 2
// 3
// Heart
// Diamond
// Spade
// Club

That includes both numbers and strings! The catch? Those numbers we see printed are actually strings. So some check such as typeof suit === 'string' is not sufficient. The solution, use Number as a boolean check:

for (const suit in Suit) {
    if (!Number(suit)) {
        console.log(suit);
    }
}

// Heart
// Diamond
// Spade
// Club

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