unexpected result while using findIndex array method

You only need to make the return inside the findIndex function (to return the index where the condition is true). When findIndex no match with some result returns -1.

    const account1 = {
      owner: 'Jonas Schmedtmann',
      movements: [200, 450, -400, 3000, -650, -130, 70, 1300],
      interestRate: 1.2, // %
      pin: 1111,
    };
    
    const account2 = {
      owner: 'Jessica Davis',
      movements: [5000, 3400, -150, -790, -3210, -1000, 8500, -30],
      interestRate: 1.5,
      pin: 2222,
    };
    
    const account3 = {
      owner: 'Steven Thomas Williams',
      movements: [200, -200, 340, -300, -20, 50, 400, -460],
      interestRate: 0.7,
      pin: 3333,
    };
    const account4 = {
      owner: 'Sarah Smith',
      movements: [430, 1000, 700, 50, 90],
      interestRate: 1,
      pin: 4444,
    };
    const accounts = [account1, account2, account3, account4];
    const index = accounts.findIndex(function (acc) {
      return acc.pin === 2222;
    });
    console.log(index);

And I prefer the code like this:

    const accounts = [
        {
          owner: 'Jonas Schmedtmann',
          movements: [200, 450, -400, 3000, -650, -130, 70, 1300],
          interestRate: 1.2, // %
          pin: 1111,
        },
        {
          owner: 'Jessica Davis',
          movements: [5000, 3400, -150, -790, -3210, -1000, 8500, -30],
          interestRate: 1.5,
          pin: 2222,
        },
        {
          owner: 'Steven Thomas Williams',
          movements: [200, -200, 340, -300, -20, 50, 400, -460],
          interestRate: 0.7,
          pin: 3333,
        },
        {
          owner: 'Sarah Smith',
          movements: [430, 1000, 700, 50, 90],
          interestRate: 1,
          pin: 4444,
        }
    ];

    const index = accounts.findIndex(function (acc) {
      return acc.pin === 2222;
    });

    console.log(index);