CODEWITHSUNDEEP
arraysstringtypescripttypestype-conversion
Anurat Chapanond
Anurat Chapanond

Reputation: 2977

typescript type return from String.concat

I have a function that converts an integer to English words. I chain the array operations to convert the number in one go. The problem I have is in reduce() I have an array concat() which should always return string[] type, however typescript compiler complains that the type returned from reduce() is string[] | string. Therefore the map() after reduce() can't be called and tsc complains 'map does not exist on type string'.

inEnglish(n: number): string {
    return n === 0? 'zero': n.toLocaleString().split(',')
            .reverse()
            .map((g, i) => Number(g)? [g, scales[i]]: g)
            .reverse()
            .reduce((g: string[], a) => g.concat(a), [])
            .map(g => Number(g)? this.under1000(Number(g)): g)
            .join('')
}

Since I know that concat() will always return type string[], how can I tell tsc that it will always be type string[] and not type string?

Upvotes: 1

Views: 848

Answers (1)

Titian Cernicova-Dragomir
Titian Cernicova-Dragomir

Reputation: 249486

The solution would be to specify the result of reduce explicitly:

var scales :string[] =[]
function inEnglish(n: number): string {
    return n === 0? 'zero': n.toLocaleString().split(',')
            .reverse()
            .map((g, i) => Number(g)? [g, scales[i]]: g)
            .reverse()
            .reduce<string[]>((g, a) => g.concat(a), [''])
            .map(g => Number(g)? this.under1000(Number(g)): g)
            .join('')
}

Upvotes: 2

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