Async function in javascript stops executing all by itself

Question

Here I have an async function in javascript that is used to fill up the sudoku board with numbers (basically its solution). I used a sleeper function between each number insertion so that the user can get a better feel of the recursion and backtracking algorithm, however after the fifteenth number gets inserted, the function stops by itself... What is going on here exactly ?

var finalInd;

function sleep() {
    console.log("happy")
    return new Promise(resolve => setTimeout(resolve, 100));
}


async function solve () {

    allowed = false;
    var empty = findEmptySpace();

    if(!empty) {
        return true;
    }

    for(let i=1; i<10; i++) {

        if(checkDuplicates(board, i, empty)) {

            board[empty[0]][empty[1]] = i;
            finalInd = (empty[0]*9) + empty[1];

            await sleep()
            funcId("board").children[finalInd].innerHTML = i;

            if(solve(board)) {
                return true;
            }
            
            board[empty[0]][empty[1]] = 0;
            
            funcId("board").children[finalInd].innerHTML = 0;
        }
    }

    funcId("board").children[0].innerHTML = board[0][0];
    return false;

}


function checkDuplicates (board, num, empty) {
    for(let i=0; i<9; i++) {
        if(board[empty[0]][i] == num && empty[1] != i) {
            return false;
        }
    }

    for(let i=0; i<9; i++) {
        if(board[i][empty[1]] == num && empty[0] != i) {
            return false;
        }
    }

    var x = Math.floor(empty[1]/3);
    var y = Math.floor(empty[0]/3);

    for(let i=(y*3); i<(y*3)+3; i++) {
        for(let j=(x*3); j<(x*3)+3; j++) {
            if(board[i][j] == num && i != empty[0] && j != empty[1]) {
                return false;
            }
        }
    }

    return true;

}


function findEmptySpace () {

    for(let i=0; i<9; i++) {
        for(let j=0; j<9; j++) {
            if(board[i][j] == 0) {
                return [i, j];
            }
        }
    }

}

Guillaume Brunerie · Accepted Answer · 2022-10-12 16:02:58Z

1

I think you forgot to await the recursive call to solve, so it will always return a promise, which it's truthy, and your function will terminate.

answered Oct 12, 2022 at 16:02

Guillaume Brunerie

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3 Comments

Nitin Over a year ago

Could you please explain the last part of what you said ? Here the only promise returned is the wait for 100ms . How does a recursive function create an issue for that, I dont really understand

Guillaume Brunerie Over a year ago

Your function solve is declared async, which means that it returns a promise. The keywords async and await are syntactic sugar for promises even if you don't write the word Promise explicitly. So solve returns a promise that resolves as a boolean. But every promise is truthy (because it's an object and every object is truthy), so if (solve(board)) {...} will always run the code inside the if.

Guillaume Brunerie Over a year ago

You can read developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… for more information.

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