Cannot read property 'includes' of undefined for JavaScript Array object

var nestedArr = [[1, 2, 3], [5, 2, 1, 4]];
var result = [];

  for (var a = 0; a < nestedArr.length; a++) { // iterate nestedArr
    for (var b = 0; b < nestedArr[a].length; b++) {
      if (nestedArr[a].includes(nestedArr[a][b])) {
        result.push(nestedArr[a][b]);
    }
  }
}
console.log(result)

removing the plus one did the job, basically you tried to access an array were no arrays were present

The program will search for an index that doesn't exist.

var nestedArr = [[1, 2, 3], [5, 2, 1, 4]];
  ...
    ...
      if (nestedArr[a+1].includes(nestedArr[a][b])) {
      // As nestedArr.length will return 2, the program will eventually  search for nestedArr[2+1 (=3)] which doesn't exist.
    }
  }
}

Bonus: Why not use for var i in nestedArr instead of using .length

Take a look at this answer.

As @Nina Scholz commented

the last index plus one does not exist

You don't need to check by includes ! Just do loop and assign using filter and map as example !!!

var nestedArr = [[1, 2, 3], [5, 2, 1, 4]];
    var result = [];
    nestedArr.filter( (val) => {
      val.map( (v) => { result.push(v); } )
    })
    console.log(result);