In JavaScript, how to select element by class?

In JavaScript, the getElementsByClassName(className) method returns an array-like object of all child elements which have the given class name. Here are some examples:

// Get all elements from the document that have the class 'center'
document.getElementsByClassName('center')

// Get all elements that have a class of 'bold', inside of an element with ID 'container'
document.getElementById('container').getElementsByClassName('bold')