All the New ES2019 Tips and Tricks

let students = 

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The ECMAScript standard has been updated yet again with the addition of new features in ES2019. Now officially available in node, Chrome, Firefox, and Safari you can also use Babel to compile these features to a different version of JavaScript if you need to support an older browser.

Let’s look at what’s new!

Object.fromEntries

In ES2017, we were introduced to Object.entries. This was a function that translated an object into its array representation. Something like this:

let students = {
  amelia: 20,
  beatrice: 22,
  cece: 20,
  deirdre: 19,
  eloise: 21
}

Object.entries(students) 
// [
//  [ 'amelia', 20 ],
//  [ 'beatrice', 22 ],
//  [ 'cece', 20 ],
//  [ 'deirdre', 19 ],
//  [ 'eloise', 21 ]
// ]

This was a wonderful addition because it allowed objects to make use of the numerous functions built into the Array prototype. Things like map, filter, reduce, etc. Unfortunately, it required a somewhat manual process to turn that result back into an object.

let students = {
  amelia: 20,
  beatrice: 22,
  cece: 20,
  deirdre: 19,
  eloise: 21
}

// convert to array in order to make use of .filter() function
let overTwentyOne = Object.entries(students).filter(([name, age]) => {
  return age >= 21
}) // [ [ 'beatrice', 22 ], [ 'eloise', 21 ] ]

// turn multidimensional array back into an object
let DrinkingAgeStudents = {}
for (let [name, age] of DrinkingAgeStudents) {
    DrinkingAgeStudents[name] = age;
}
// { beatrice: 22, eloise: 21 }

Object.fromEntries is designed to remove that loop! It gives you much more concise code that invites you to make use of array prototype methods on objects.

let students = {
  amelia: 20,
  beatrice: 22,
  cece: 20,
  deirdre: 19,
  eloise: 21
}

// convert to array in order to make use of .filter() function
let overTwentyOne = Object.entries(students).filter(([name, age]) => {
  return age >= 21
}) // [ [ 'beatrice', 22 ], [ 'eloise', 21 ] ]

// turn multidimensional array back into an object
let DrinkingAgeStudents = Object.fromEntries(overTwentyOne); 
// { beatrice: 22, eloise: 21 }

It is important to note that arrays and objects are different data structures for a reason. There are certain cases in which switching between the two will cause data loss. The example below of array elements that become duplicate object keys is one of them.

let students = [
  [ 'amelia', 22 ], 
  [ 'beatrice', 22 ], 
  [ 'eloise', 21], 
  [ 'beatrice', 20 ]
]

let studentObj = Object.fromEntries(students); 
// { amelia: 22, beatrice: 20, eloise: 21 }
// dropped first beatrice!

When using these functions make sure to be aware of the potential side effects.

Support for Object.fromEntries

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Array.prototype.flat

Multi-dimensional arrays are a pretty common data structure to come across, especially when retrieving data. The ability to flatten it is necessary. It was always possible, but not exactly pretty.

Let’s take the following example where our map leaves us with a multi-dimensional array that we want to flatten.

let courses = [
  {
    subject: "math",
    numberOfStudents: 3,
    waitlistStudents: 2,
    students: ['Janet', 'Martha', 'Bob', ['Phil', 'Candace']]
  },
  {
    subject: "english",
    numberOfStudents: 2,
    students: ['Wilson', 'Taylor']
  },
  {
    subject: "history",
    numberOfStudents: 4,
    students: ['Edith', 'Jacob', 'Peter', 'Betty']
  }
]

let courseStudents = courses.map(course => course.students)
// [
//   [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
//   [ 'Wilson', 'Taylor' ],
//   [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
// ]

[].concat.apply([], courseStudents) // we're stuck doing something like this

In comes Array.prototype.flat. It takes an optional argument of depth.

let courseStudents = [
  [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
  [ 'Wilson', 'Taylor' ],
  [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
]

let flattenOneLevel = courseStudents.flat(1)
console.log(flattenOneLevel)
// [
//   'Janet',
//   'Martha',
//   'Bob',
//   [ 'Phil', 'Candace' ],
//   'Wilson',
//   'Taylor',
//   'Edith',
//   'Jacob',
//   'Peter',
//   'Betty'
// ]

let flattenTwoLevels = courseStudents.flat(2)
console.log(flattenTwoLevels)
// [
//   'Janet',   'Martha',
//   'Bob',     'Phil',
//   'Candace', 'Wilson',
//   'Taylor',  'Edith',
//   'Jacob',   'Peter',
//   'Betty'
// ]

Note that if no argument is given, the default depth is one. This is incredibly important because in our example that would not fully flatten the array.

let courseStudents = [
  [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
  [ 'Wilson', 'Taylor' ],
  [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
]

let defaultFlattened = courseStudents.flat()
console.log(defaultFlattened)
// [
//   'Janet',
//   'Martha',
//   'Bob',
//   [ 'Phil', 'Candace' ],
//   'Wilson',
//   'Taylor',
//   'Edith',
//   'Jacob',
//   'Peter',
//   'Betty'
// ]

The justification for this decision is that the function is not greedy by default and requires explicit instructions to operate as such. For an unknown depth with the intention of fully flattening the array the argument of Infinity can be used.

let courseStudents = [
  [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
  [ 'Wilson', 'Taylor' ],
  [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
]

let alwaysFlattened = courseStudents.flat(Infinity)
console.log(alwaysFlattened)
// [
//   'Janet',   'Martha',
//   'Bob',     'Phil',
//   'Candace', 'Wilson',
//   'Taylor',  'Edith',
//   'Jacob',   'Peter',
//   'Betty'
// ]

As always, greedy operations should be used judiciously and are likely not a good choice if the depth of the array is truly unknown.

Support for Array.prototype.flat

Array.prototype.flatMap

With the addition of flat we also got the combined function of Array.prototype.flatMap. We've actually already seen an example of where this would be useful above, but let's look at another one.

What about a situation where we want to insert elements into an array. Prior to the additions of ES2019, what would that look like?

let grades = [78, 62, 80, 64]

let curved = grades.map(grade => [grade, grade + 7])
// [ [ 78, 85 ], [ 62, 69 ], [ 80, 87 ], [ 64, 71 ] ]

let flatMapped = [].concat.apply([], curved) // now flatten, could use flat but that didn't exist before either
// [
//  78, 85, 62, 69,
//  80, 87, 64, 71
// ]

Now that we have Array.prototype.flat we can improve this example slightly.

let grades = [78, 62, 80, 64]

let flatMapped = grades.map(grade => [grade, grade + 7]).flat()
// [
//  78, 85, 62, 69,
//  80, 87, 64, 71
// ]

But still, this is a relatively popular pattern, especially in functional programming. So having it built into the array prototype is great. With flatMap we can do this:

let grades = [78, 62, 80, 64]

let flatMapped = grades.flatMap(grade => [grade, grade + 7]);
// [
//  78, 85, 62, 69,
//  80, 87, 64, 71
// ]

Now, remember that the default argument for Array.prototype.flat is one. And flatMap is the equivalent of combing map and flat with no argument. So flatMap will only flatten one level.

let grades = [78, 62, 80, 64]

let flatMapped = grades.flatMap(grade => [grade, [grade + 7]]);
// [
//   78, [ 85 ],
//   62, [ 69 ],
//   80, [ 87 ],
//   64, [ 71 ]
// ]

Support for Array.prototype.flatMap

String.trimStart and String.trimEnd

Another nice addition in ES2019 is an alias that makes some string function names more explicit. Previously, String.trimRight and String.trimLeft were available.

let message = "   Welcome to CS 101    "
message.trimRight()
// '   Welcome to CS 101'
message.trimLeft()
// 'Welcome to CS 101   '
message.trimRight().trimLeft()
// 'Welcome to CS 101'

These are great functions, but it was also beneficial to give them names that more aligned with their purpose. Removing starting space and ending space.

let message = "   Welcome to CS 101    "
message.trimEnd()
// '   Welcome to CS 101'
message.trimStart()
// 'Welcome to CS 101   '
message.trimEnd().trimStart()
// 'Welcome to CS 101'

Support for String.trimStart and String.trimEnd

Optional catch binding

Another nice feature in ES2019 is making an argument in try-catch blocks optional. Previously, all catch blocks passed in the exception as a parameter. That meant that it was there even when the code inside the catch block ignored it.

try {
  let parsed = JSON.parse(obj)
} catch(e) {
  // ignore e, or use
  console.log(obj)
}

This is no longer the case. If the exception is not used in the catch block, then nothing needs to be passed in at all.

try {
  let parsed = JSON.parse(obj)
} catch {
  console.log(obj)
}

This is a great option if you already know what the error is and are looking for what data triggered it.

Support for Optional Catch Binding

Function.toString() changes

ES2019 also brought changes to the way Function.toString() operates. Previously, it stripped white space entirely.

function greeting() {
  const name = 'CSS Tricks'
  console.log(`hello from ${name}`)
}

greeting.toString()
//'function greeting() {\nconst name = \'CSS Tricks\'\nconsole.log(`hello from ${name} //`)\n}'

Now it reflects the true representation of the function in source code.

function greeting() {
  const name = 'CSS Tricks'
  console.log(`hello from ${name}`)
}

greeting.toString()
// 'function greeting() {\n' +
//  "  const name = 'CSS Tricks'\n" +
//  '  console.log(`hello from ${name}`)\n' +
//  '}'

This is mostly an internal change, but I can’t help but think this might also make the life easier of a blogger or two down the line.

Support for Function.toString

And there you have it! The main feature additions to ES2019.

There are also a handful of other additions that you may want to explore. Those include:

• Symbol description
• Sort stability
• ECMAScript as JSON superset
• JSON.stringify

Happy JavaScript coding!

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