How to pass an enum parameter from a loop?

Question

Consider the code below. I assumed that i could loop through an enum and pass an instance of it to a function that takes the enum as parameter.

However, it seems that let color in Colors produced color of type string instead of type Colors. Am I missing a cast somewhere? Why doesn't enum loop produce a value of said Enum?

enum Colors {
    Red = "Red",
    Green = "Green",
    Yellow = "Yellow"
}

for (let color in Colors) {
  doStuff(color);
}

function doStuff(mycolor: Colors) {
  console.log(mycolor)
}

Ali Habibzadeh · Accepted Answer · 2019-04-08 22:45:21Z

3

Use Object.values as for loop just treats it as iterating over an object

enum Colors {
  Red = "Red",
  Green = "Green",
  Yellow = "Yellow"
}

function doStuff(mycolor: Colors): void {
  console.log(mycolor);
}

Object.values(Colors).forEach(color => doStuff(color));

I guess Colours are only the values and not the enum entries, as you can use this too:

Object.entries(Colors).forEach((color: [string, Colors]) => doStuff(color[1]));

Also bear in mind that for loops generally are not good for things that don't have if or switch statement. Interestingly if you do add deeper logic to the for loop, TypeScript does determine the type:

for (const color in Colors) {
  if (color === Colors.Green) { 
    // Now the cat is either dead or alive 
    doStuff(color);
  }
}

Object.entries is still new so ensure that your tsconfig.json is referencing es2017 as an entry for compilerOptions/lib.

edited Apr 8, 2019 at 22:45

answered Apr 8, 2019 at 22:19

Ali Habibzadeh

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4 Comments

Ali Habibzadeh Over a year ago

Thanks for reminding me. I will edit the answer to include this

Ali Habibzadeh Over a year ago

To really get the value of TS enable compilerOptions/strict: true as well.

AngryHacker Over a year ago

Well, that opened a can of worms.

AngryHacker Over a year ago

Excellent. I'd like to add one small thing. in tsconfig.json, it's necessary to add es2017 to compilerOptions/lib entry.

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