您可以检查未定义的 并使用'-' ，否则为值，并使用 Array＃map 获取新数组。

You could check for undefined and take '-', otherwise the value and use Array#map for getting a new array.

var array = [1, 2, undefined, undefined, 7, undefined],
    result = array.map(v => v === undefined ? '-' : v);
    
console.log(result);

对于稀疏数组，您需要迭代所有索引并检查值。

For a sparse array, you need to iterate all indices and check the values.

var array = [1, 2, , , 7, ,],
    result = Array.from(array, v => v === undefined ? '-' : v);
    
console.log(result);