I have an interface like this

interface FooProps {
    cat: string[],
    kitten: number[],
}

Is there any way to tell TypeScript that kitten and cat must be the same length? Note, the length varies, depending on where the interface FooProps is used. But, whenever I use FooPros, the length of cat and kitten must be the same.

>Solution :

interface FooProps {
  cat: string[];
  kitten: number[];
}

// If cat and kitten are of fixed same length then
// the easiest way would be to define them as supple

interface FooPropsAsTupple {
  cat: [string, string];
  kitten: [number, number];
}

const catsTupples: FooPropsAsTupple = {
  cat: ["", ""],
  kitten: [1, 2],
};

// however if they are dynamic, then you need to validate
// using a class (There are other ways too but this is easy)

class Felines implements FooProps {
  constructor(props: FooProps) {
    if (props.cat.length !== props.kitten.length) {
      throw new Error(`Cat and kitten must have the same length`);
    }
    const { cat, kitten } = props;
    this.cat = cat;
    this.kitten = kitten;
  }
  public cat!: string[];
  public kitten!: number[];
}

// Error: Cat and kitten must have the same length
new Felines({ cat: ["", "", ""], kitten: [1, 2] });

You may even combine both approach in order to get type completion but also actual safety by throwing error.