We can use Array.sort method to sort this array.
var array = ["ab", "abcdefgh", "abcd"];
array.sort(function(a, b){return b.length - a.length});
console.log(JSON.stringify(array, null, '\t'));For ascending sort order:
a.length - b.lengthFor descending sort order:
b.length - a.length
Attention: not all browsers can understand ES6 code!
In ES6 we can use an arrow function expressions.
let array = ["ab", "abcdefgh", "abcd"];
array.sort((a, b) => b.length - a.length);
console.log(JSON.stringify(array, null, '\t'));