Array.prototype.some();

some-adding_2different_classes.js

// Array.prototype.some()

var tasks = [
    {
        title: 'Do laundry',
        completed: true
    },
    {
        title: 'Feed the cat',
        completed: true
    },
    {
        title: 'Watch the array lessons on egghead.io',
        completed: true
    }
];

var list = document.querySelector('.task-list'); // select the <ul> element
list.classList.add(
        tasks.some(task => task.completed === false) // if any task has the completed value false => return true => add class uncompleted
        ? 'task-list--uncompleted'
        : 'task-list--completed'              // else if task.completed === false => returns false => add class 'completed'
);

list.innerHTML = tasks
   .map(x => x.completed ? `<s>${x.title}</s>` : x.title)
   .map(x => `<li>${x}</li>`)
   .join('');

some.js

// Array.prototype.some() is used when you just want a quick query on the array when you just want a YEs or No answer.
// Returns true or false

// Basic example 

var items = [1, 2, 3, 4, 5];

var hasThree = items.some(item => (item === 3)); // as soon as the wanted item is found, the loop stops and returns true
console.log(hasThree); // true