这会有用的

var arr = [2,3,4];
var largest = 0;

arr.forEach(function(elem){
  if(largest < elem) 
  largest = elem;
});

console.log(largest);

但你为什么要这么做为什么不使用Max？

var largest = Math.max.apply(Math, arr);

为了覆盖数组中的负数，将largest指定为数组中的第一个元素是安全的。

var arr = [2,3,4];
var largest = arr[0];
 
arr.forEach(function(elem){
  if(largest < elem) 
  largest = elem;
});
console.log(largest);

你可以简单地使用spread operator：

var arr = [2,3,4];
var largest = Math.max(...arr); 
console.log(largest);

如果将console.log(largest);放在forEach循环之外，它将打印数组中最大的数字。

var
    a = [2,3,4,2,1,9],
    largest = 0;

a.forEach(function(value){
    console.log('current array element value: ' + value + '.  Is it larger than the largest?', (largest > value));
    if (largest < value) largest = value;
    console.log('largest value now is: ', largest);
});
console.log('largest is changed here, because it was not redefined inside the function of the forEach method: ', largest, '.\n But it was defined within its context.');

function greatest(array) {
    if(array == undefined){
        console.log('There are no elements in array.');
    }

    const max = array.reduce(function(first, next){
        return first > next ? first : next; 
    });

    console.log(`Maximum element is ${max}`);
}