Edit in JSFiddle

var tasks = [
    { task: 'task1', isDone: true, tags: ['project1', 'memo'] },
    { task: 'task2', isDone: false, tags: ['project1', 'memo'] },
    { task: 'task3', isDone: false, tags: ['project2', 'asap'] }
];

var data = JSON.stringify(tasks);
var a = document.createElement('a');
a.textContent = 'export'
a.download = 'tasks.json';
a.href = window.URL.createObjectURL(new Blob([data], { type: 'text/plain' }));
a.dataset.downloadurl = ['text/plain', a.download, a.href].join(':');

var exportLink = document.getElementById('export-link');
exportLink.appendChild(a);

<span id="export-link"></span>