I am new to TypeScript and I don't understand what I need to do to fix the line that generates the TS7015 error (referencing an enum member using a string variable) because the line immediately following that does not error (referencing an enum member using a string literal):
enum State {
Happy = 0,
Sad = 1,
Drunk = 2
}
function Emote(enumKey:string) {
console.log(State[enumKey]); // error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
console.log(State["Happy"]); // no error
}
"noImplicitAny": true
is set in the project's tsconfig.json
the error is detected
"noImplictAny": false
is set in the project's tsconfig.json
no error is detected
I'm compiling with "ntypescript": "^1.201603060104.1"
I'm now compiling with "tsc": "1.8.10"
C:>npm install -g typescript
`-- [email protected]
Verifying installation:
C:\>tsc --version
Version 1.8.10
Here's my tsconfig.json
file:
{
"compileOnSave": true,
"compilerOptions": {
"target": "ES5",
"module": "System",
"moduleResolution": "node",
"emitDecoratorMetadata": true,
"experimentalDecorators": true,
"removeComments": true,
"noImplicitAny": true,
"sourceMap": true,
"mapRoot": "map/",
"diagnostics": true
},
"exclude": [
"node_modules",
"typings"
]
}
Here's the compiler output:
C:\>tsc
test.ts(8,17): error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
If you're using TypeScript 2.1+, you can change enumKey
's type to keyof typeof State
, like this:
function Emote(enumKey: keyof typeof State) {...}
or, if the function's input is required to be a string
, this:
var state : State = State[enumKey as keyof typeof State];
Explanation:
Because enumKey
is an arbitrary string
, TypeScript doesn't know whether enumKey
is the name of a member of State
, so it generates an error. TypeScript 2.1 introduced the keyof
operator which returns a union of the known, public property names of a type. Using keyof
allows us to assert that the property is indeed in the target object.
However, when you create an enum, TypeScript actually produces both a type (which is typically a subtype of number
) and a value (the enum object that you can reference in expressions). When you write keyof State
, you're actually going to get a union of the literal property names of number
. To instead get the property names of the enum object, you can use keyof typeof State
.
Sources:
https://github.com/Microsoft/TypeScript/issues/13775#issuecomment-276381229 https://www.typescriptlang.org/docs/handbook/enums.html#enums-at-compile-time
I suspect it has to do with TS 1.8.x's new support for string literals in these situations. TS happens to know that "Happy" is a valid string index, but it doesn't know whether enumKey
will be or not. You can fix it by casting it to an <any>
, like so:
function Emote(enumKey:string) {
console.log(State[enumKey]); // error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
console.log(State["Melancholy"]); // error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
console.log(State["Happy"]); // no error
console.log(State[<any>enumKey]); // no error
console.log(State[<any>"Melancholy"]); // no error
}
(BTW, I think this is new: I couldn't reproduce this error with 1.8.9, but as soon as I upgraded to 1.8.10, I could.)
Also interestingly, I would have expected this to work without the error, but it doesn't:
function TypedEmote(enumKey:'Happy'|'Sad'|'Drunk'){
console.log(State[enumKey]);
}
Must be something about the TS spec I don't understand, or perhaps they just haven't gotten around to fixing that bit yet.
You can prevent this error with the compiler option without loosing the whole strict null checks
"suppressImplicitAnyIndexErrors": true
https://www.typescriptlang.org/tsconfig#suppressImplicitAnyIndexErrors
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