array = [['10','8'],['11','4'],['18','7'],['7','6'],['8','7'],['9','5']]
function sortNumber(a,b){return a[0]-b[0]}
let newArray = array.sort(sortNumber)

newArray 应该就是你要的结果了

var arr = [['10','8'],['11','4'],['18','7'],['7','6'],['8','7'],['9','5']]
arr.sort(function(a, b) {
    return a[0]-b[0]
})

你这个二维数组的排列的结果，好像是用的数组中的每个元素（数组）中的第一个下标的元素去比较的，那就转化成一个一维数组排序不就不好了吗？一维数组排好序，再将原二维数组利用index对应上一维数组排序。
逻辑实现思路：
var twoArray = [['10','8'],['11','4'],['18','7'],['7','6'],['8','7'],['9','5']];

    var oneArray = [];
    twoArray.map(function (item ,index) {
        oneArray.push({value: item[0], index: index});
    });
    console.log(oneArray);
    function sortNumber (a, b) {
        return a.value - b.value;
    }
    console.log(oneArray.sort(sortNumber));
    // 此时的oneArray已排好序
    var newTwoArray = [];
    oneArray.map(function (item) {
        newTwoArray.push(twoArray[item.index]);
    });
    console.log(newTwoArray);// 即你要的排序

var groupNum = 3;

var arr1 = [['10','8'],['11','4'],['18','7'],['7','6'],['8','7'],['9','5']];

var arr2 = [];

for (var i = arr1.length; i > 0 ; i -= groupNum) {
  arr2.push(arr1.slice(i - groupNum, i));
}

console.log(arr2) // [['7','6'],['8','7'],['9','5']，['10','8'],['11','4'],['18','7']]