First the really easy advice:

• use strict mode https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode
• use const instead of let for stuff you know you're not supposed to change (symbol, player1, player2); declaring it const helps you keep some assumptions later
• use === instead of == generally; I think the only legitimate use for == is for using it with null when you allow undefined or results of typeof if you feel like you're a JS human minifier :)
• prefer camel case since JS already does it and it would look weird to have two casing strategies in the same codebase
• use better naming in general: I'm talking about array in function checkforOwin(array) - array is extremely generic (what array?!?) I would call it board so I know what I'm dealing with; the fact that it's implemented as an array is a detail
• I'd also like a function named checkforOwin to just check and return true/false than exit the process xD
• who's O in this same function name? I thought we're comparing with the variable player1; stick to one or the other, not both; you'd have to update quite a bit of code if you change the initial statement of let player1 = "X"

...and the more involved:

• I would use nested arrays for the board because I can't easily picture what board[7] is; but I can picture what board[2][1] is
• those really long if statements look very repetitive; they look a lot like patterns on a board so instead let's turn that code into data so we can manage it easier; we can rewrite the patterns like this:

const isOWinner = [
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8],
  [0, 4, 8],
// ...
].some((indices) => indices.every((index) => board[index] === player1))

That big condition blob in your if statements can be rewritten like this. Because it's an array you can store it somewhere; load it; merge with other patterns or extend or whatever you can do with arrays. Code that looks like data should be represented as data

First the really easy advice:

• use strict mode https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode
• use const instead of let for stuff you know you're not supposed to change (symbol, player1, player2); declaring it const helps you keep some assumptions later
• use === instead of == generally; I think the only legitimate use for == is for using it with null when you allow undefined or results of typeof if you feel like you're a JS human minifier :)
• prefer camel case since JS already does it and it would look weird to have two casing strategies in the same codebase
• use better naming in general: I'm talking about array in function checkforOwin(array) - array is extremely generic (what array?!?) I would call it board so I know what I'm dealing with; the fact that it's implemented as an array is a detail
• I'd also like a function named checkforOwin to just check and return true/false than exit the process xD
• who's O in this same function name? I thought we're comparing with the variable player1; stick to one or the other, not both; you'd have to update quite a bit of code if you change the initial statement of let player1 = "X"

...and the more involved:

• I would use nested arrays for the board because I can't easily picture what board[7] is; but I can picture what board[2][1] is
• those really long if statements look very repetitive; they look a lot like patterns on a board so instead let's turn that code into data so we can manage it easier; we can rewrite the patterns like this:

const isOWinner = [
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8],
  [0, 4, 8],
// ...
].some((indices) => indices.every((index) => board[index] === player1))

That big condition blob in your if statements can be rewritten like this. Because it's an array you can store it somewhere; load it; merge with other patterns or extend or whatever you can do with arrays. Code that looks like data should be represented as data. In fact let's use the same data in both functions and merge them into one:

function checkForWin (board, player) {
  const isWinner = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 4, 8],
    // ...
  ].some((indices) => indices.every((index) => board[index] === player))

  if (isWinner) {
    console.log(player + 'wins')
    process.exit()
  }
}

And now you just call checkForWin(board, player1) or checkForWin(board, player2)

First the really easy advice:

• use strict mode https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode
• use const instead of let for stuff you know you're not supposed to change (symbol, player1, player2); declaring it const helps you keep some assumptions later
• use === instead of == generally; I think the only legitimate use for == is for using it with null when you allow undefined or results of typeof if you feel like you're a JS human minifier :)
• use better naming in general and prefer camel case since JS already does it and it would look weird to have two casing strategies in the same codebase; I'm also talking about array in function checkforOwin(array) - array is extremely generic (what array?!?) I would call it board so I know what I'm dealing with; the fact that it's implemented as an array is a detail; I'd also like a function named like that to just check and return true/false than exit the process xD

...and the more involved:

• I would use nested arrays for the board because I can't easily picture what board[7] is; but I can picture what board[2][1] is
• those really long if statements look very repetitive; they look a lot like patterns on a board so instead let's turn that code into data so we can manage it easier; we can rewrite the patterns like this:

const isOWinner = [
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8],
  [0, 4, 8],
// ...
].some((indices) => indices.every((index) => board[index] === player1))

That big condition blob in your if statements can be rewritten like this. Because it's an array you can store it somewhere; load it; merge with other patterns or extend or whatever you can do with arrays. Code that looks like data should be represented as data

First the really easy advice:

• use strict mode https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode
• use const instead of let for stuff you know you're not supposed to change (symbol, player1, player2); declaring it const helps you keep some assumptions later
• use === instead of == generally; I think the only legitimate use for == is for using it with null when you allow undefined or results of typeof if you feel like you're a JS human minifier :)
• prefer camel case since JS already does it and it would look weird to have two casing strategies in the same codebase
• use better naming in general: I'm talking about array in function checkforOwin(array) - array is extremely generic (what array?!?) I would call it board so I know what I'm dealing with; the fact that it's implemented as an array is a detail
• I'd also like a function named checkforOwin to just check and return true/false than exit the process xD
• who's O in this same function name? I thought we're comparing with the variable player1; stick to one or the other, not both; you'd have to update quite a bit of code if you change the initial statement of let player1 = "X"

...and the more involved:

• I would use nested arrays for the board because I can't easily picture what board[7] is; but I can picture what board[2][1] is
• those really long if statements look very repetitive; they look a lot like patterns on a board so instead let's turn that code into data so we can manage it easier; we can rewrite the patterns like this:

const isOWinner = [
  [0, 1, 2],
  [3, 4, 5],
  [6, 7, 8],
  [0, 4, 8],
// ...
].some((indices) => indices.every((index) => board[index] === player1))

That big condition blob in your if statements can be rewritten like this. Because it's an array you can store it somewhere; load it; merge with other patterns or extend or whatever you can do with arrays. Code that looks like data should be represented as data