closestPath.js

const array = [
    ['a', ['m', 'n', 'o']],
    ['b', ['s', 't', 'u']],
    ['d', ['e', 'f', 'g']],
]

function getShortestPath(target, array){
    let shortest = array[0];
    let shortestDistance = Infinity;
    for(let i = 0; i < array.length; i++){
        let distance = array[i][1].indexOf(target);
        if(distance !== -1){
            if(distance < shortestDistance){
                shortest = array[i];
                shortestDistance = distance;
            }
        }
    }
    return `${shortest[0]} => ${target}` ;
}

console.log(getShortestPath('f', array)); // d => f