fn.ts

function formula_xfyz (x: string | symbol, y: string | symbol, z: string | symbol, f: '+' | '-' | '*' | '/' | '^') {
    // function fn (it: { [x]: number, [y]: number }): { [z]: number};
    // function fn (it: { [x]: number, [z]: number }): { [y]: number};
    // function fn (it: { [y]: number, [z]: number }): { [x]: number};
    function fn (it: any): any {
        const g = ({
            '+': [  (x: number, y: number): number /* z */ => x + y,
                    (x: number, z: number): number /* y */ => z - x,
                    (y: number, z: number): number /* x */ => z - y ],
            '-': [  (x: number, y: number): number /* z */ => x - y,
                    (x: number, z: number): number /* y */ => x - z,
                    (y: number, z: number): number /* x */ => z + y ],
            '*': [  (x: number, y: number): number /* z */ => x * y,
                    (x: number, z: number): number /* y */ => z / x,
                    (y: number, z: number): number /* x */ => z / y ],
            '/': [  (x: number, y: number): number /* z */ => x / y,
                    (x: number, z: number): number /* y */ => x / z,
                    (y: number, z: number): number /* x */ => z * y ],
            '^': [  (x: number, y: number): number /* z */ => x ** y,
                    (x: number, z: number): number /* y */ => Math.log(z) / Math.log(x),
                    (y: number, z: number): number /* x */ => z ** (1 / y) ],
        })[f];

        if (it.hasOwnProperty(x)) {
            if (it.hasOwnProperty(y)) return (x: number, y: number) => ({ [z]: g[0](x, y) });
            if (it.hasOwnProperty(z)) return (x: number, z: number) => ({ [y]: g[1](x, z) });
        }
        
        if (it.hasOwnProperty(y)) {
            if (it.hasOwnProperty(x)) return (x: number, y: number) => ({ [z]: g[0](x, y) });
            if (it.hasOwnProperty(z)) return (y: number, z: number) => ({ [x]: g[2](y, z) });
        }

        if (it.hasOwnProperty(z)) {
            if (it.hasOwnProperty(x)) return (x: number, z: number) => ({ [y]: g[1](x, z) });
            if (it.hasOwnProperty(y)) return (y: number, z: number) => ({ [x]: g[2](y, z) });
        }   
    }
    return fn;
}