CODEWITHSUNDEEP
typescript
Miguel Moura
Miguel Moura

Reputation: 39374

Parse String to Enum in Typescript using Strict mode enabled

When using TypeScript and strict enabled I have the enum:

export enum Policy {
  Admin,
  Editor
}

Then on a class I have:

policy?: Policy;

setPolicy(policy: string) {
  this.policy = Policy[policy];
}

I need to define the policy variable from a string.

When using strict and try to compile I get the error:

Element implicitly has an 'any' type because index expression is not of type 'number'.

How can I solve this?

Upvotes: 1

Views: 311

Answers (1)

jcalz
jcalz

Reputation: 327994

The enum object named Policy is not known to have values at every string key; only the specific string literals "Admin" and "Editor" can be safely used. I assume you're not trying to support someone calling setPolicy("RandomString"), right? So one way to fix this is to only allow the policy argument to be the union of those types:

setPolicy(policy: "Admin" | "Editor") {
    this.policy = Policy[policy];
}

Of course you don't want to have to edit that line every time you add or otherwise modify the keys of Policy. So you can use the keyof type operator to get the union of keys from the type of the Policy object. Note that the type of the Policy object is not Policy, but typeof Policy which uses the typeof type operator. See this q/a for a long rant explanation about the difference between types and values in TypeScript. So you can do this:

setPolicy(policy: keyof typeof Policy) {
    this.policy = Policy[policy];
}

The type keyof typeof Policy is the same as "Admin" | "Editor", as you can see by inspecting it:

new Foo().setPolicy("Admin");
// setPolicy(policy: "Admin" | "Editor"): void

Playground link to code

Upvotes: 3

Related Questions