CODEWITHSUNDEEP
javascriptflowtype
nvie
nvie

Reputation: 376

How to express object return type based on input value in Flow?

What's the correct syntax to use for the following (maximally simplified) function:

function f(field: string): ??? {
    return {[field]: 42};
}

The output shape of this function is always:

f('foo') => {foo: 42}
f('blah') => {blah: 42}
f('a-okay') => {"a-okay": 42}
f('') => {"": 42}

Is there a syntax or a special $Utility function available to help me express the output type for this function? Is it possible at all?

For the record, I'm never invoking this function with a dynamic value, i.e.

let x = random_string();
f(x)

Instead, I'm only ever using this function with string literals (constants), like in the examples above.

Upvotes: 3

Views: 601

Answers (1)

trincot
trincot

Reputation: 350290

You cannot require the result object to have a specific key that is only known when the function is called, but you can do a type check with an indexer property:

function f(field: string): { [string]: number } {
    return {[field]: 42};
}

Upvotes: 1

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