Broken Arrows, Default Parameters

December 05, 2018

1 minute read

It's 1996. John Travolta and Christian Slater are at the height of their powers. What's better than a cheesy action flick? Okay, this post isn't about Broken Arrow... it's another post about JavaScript!

Arrow Functions

The old school of the old school (how we used to define functions back in the day):

function add(a, b) {
    return a + b;

The new hotness for functions that ES2015 gave us:

let add = (a, b) => a + b;
// You must declare your new arrow function as a variable: let add
// Parameters of function are defined next: = (a,b)
// Arrow function is almost equivalent to function: =>
// No return statement necessary if only a single line: a + b;

Arrow functions are syntactic sugar, making function declaration more concise.

There are some important catches or gotchas to remember with arrow functions though:

  • Arrow functions do NOT get their own "this" keyword
  • Within an arrow function, "this" is the enclosing context you find the arrow function in
  • For the above reason, it's a best practice to never use arrow functions as methods on objects

Default Parameters

Default parameters are commonly found in other languages, but for JavaScript it's a new trick. They help save on extra code in a function typically used to handle instances where a function is called without parameters.

The old school rudimentary way to handle undefined parameters being passed:

function add(a, b) {
    if(a === undefined) {
        a = 0;
    if(b === undefined) {
        b = 0;
    return a + b;

The new hotness that saves a lot of code duplication:

function add(a=0, b=0) {
    return a + b;

So what happens now? Well, if you call add() without parameters, now you get 0 instead of Not a Number (NaN). As you can see, this function just got a lot cleaner and it still protects against undefined parameters being called!