TypeScript conditional return value type?

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Typescript will not infer different return types based on type guards in the function. You can however define multiple function signatures for let the compiler know the links between input parameter types and result type:

function ff(x: boolean): boolean;
function ff(x: string): string;

// Implementation signature, not publicly visible
function ff(x: boolean | string): boolean | string {
    return typeof x === 'boolean' ? true : 'str'
}

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