undefined.md

Checking if a value is undefined in Javascript

1. Use typeof

let a;
console.log(typeof a === "undefined"); // returns true

2. Check if a property exists

let favouriteShirt = {
  brand: "Superdry"
}

favouriteShirt.colour // undefined
favouriteShirt.styles[0] // TypeError: Cannot read property '0' of undefined

// Use in operator
const colour = "colour" in favouriteShirt ? favouriteShirt.colour : "blue";

// De-construct with defaults
const {styles = ["round neck", "v neck", "deep v"] } = favouriteShirt;

3. Always initialise your variables

const x = null;
let y = "stuff";

4. Use default values to your functions or objects

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

let default = {
  fontSize: 12,
  color: "black",
  paddingTop: 3
};

let custom = {
  fontSize: 18
};

let object = {
  ...default,
  ...custom
};

object.fontSize // 18
object.paddingTop // 3

5. Object.is

let someObject;
Object.is(someObject, undefined);

6. Arrays

let array

if (array && Array.isArray(array)) {

}

Conclusion

I've been badly burned by these undefined variables in javascript. I could easily prevent them by doing the stuff I listed above. In my case using the in operator, assigning default values are the ones I needed to prevent it from stuffing up. Fuck! Very simple but effective.

These pointers are directly taken from https://dmitripavlutin.com/7-tips-to-handle-undefined-in-javascript/. Don't sue me. 🚜