How to determine the type of a variable depending on the type of another?

Question

typescript

There is a code for ts:

interface ModelOptions {
  value: [number, number] | number
  range: boolean
}

How to set value to type [number, number] if range === true and vice versa?

Answer 1

As close as possible to the desired is Discriminating Unions

interface ModelOptionRange {
  value: [ number, number ];
  range: true;
}

interface ModelOptionValue {
  value: number;
  range: false;
}

type ModelOption = ModelOptionRange | ModelOptionValue;

function check(v: number, cond: ModelOption): boolean {
  if (cond.range) {
    // тут cond точно типа ModelOptionRange
    const [ min, max ] = cond.value;
    return (min <= v && v <= max);
  } else {
    // а тут ModelOptionValue
    return v === cond.value;
  }
}

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