Q:

Underscore in JavaScript

What does the underscore after "state.triggeredTests =" mean:
mutations: {

setTriggeredTest(state, data) {

  state.triggeredTests = _
    .chain(data)
    .forEach((item) => {
      item.dateFormatted = moment(item.date).format('DD MMMM YYYY');
      item.explanationTest = testMapping.get(item.test);
    })
    // .sortedUniqBy('explanationTest')
    .orderBy('date')
    .groupBy('date')
    .value();
},

A:

Its a library of function/utilities/helpers which generally can be either lodash or underscore

A:

_ is the completely valid identifier in JS. _  global variable is being traditionally used by the underscore library and its clones as a namespace. 
P.S.: $ is the valid identifier as well, and it's being used by jQuery or similar libraries.