Can I avoid a type predicate or type assertion in this code?

[echo trench]

bawdyinkslinger#0

Playground Link

Preview:```ts
const logLevels = ['error', 'warn', 'info', 'debug'] as const;
export type LogLevel = (typeof logLevels)[number];

declare function getLogLevelFromEnvironmentVariable(): LogLevel; // will this implementation require a type predicate or type assertion? e.g.:
...```

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[iron elk]

!ts

[echo trench]

const logLevels = ['error', 'warn', 'info', 'debug'] as const;
export type LogLevel = (typeof logLevels)[number];

declare function getLogLevelFromEnvironmentVariable(): LogLevel; // will this implementation require a type predicate or type assertion? e.g.:

const isValidLogLevel = (input: string): input is LogLevel => {
  return logLevels.includes(input /* do I need a type assertion? */ as LogLevel);
};```

[iron elk]

I thought I might be able to use in, but I just noticed that the property name must be a string literal 😦 https://www.typescriptlang.org/docs/handbook/2/narrowing.html#the-in-operator-narrowing

[smoky summit]

how would you use in anyway?

!:includes

[echo trench]

retsam19#0

`!retsam19:includes`:

The type of Array.prototype.includes assumes that you're using the string to check something about the array. If you're trying to do the reverse, a modified type signature is useful:

function includes<S extends string>(haystack: readonly S[], needle: string): needle is S {
    const _haystack: readonly string[] = haystack;
    return _haystack.includes(needle)
}

declare const x: string;
if(includes(["a", "b", "c"], x)) {
    // x is "a" | "b" | "c"
}

[iron elk]

if (environmentVariableValue in LogLevel) {}

[smoky summit]

but the keys of LogLevel are 0, 1, 2 and 3

oh the type

LogLevel isnt a value so you can't do that anyway

[iron elk]

but...

the documentation says:

For example, with the code: "value" in x. where "value" is a string literal and x is a union type.

[smoky summit]

all types get erased at compile time

[smoky summit]

the docs are misleading

x is a variable with a union type

[iron elk]

lol

[iron elk]

In this case, it's literally just wrong, isn't it?

[smoky summit]

prolly yeah

[iron elk]

haha, okay, thanks

that includes is pretty clever, thanks