Idiomatic JavaScript

// safely extract a propertiy value from an object, gets undefined if obj is not valid or obj has no propname
let prop = obj && obj[propName]

// call func on prop if func is valid, and use the result returned by func as the value. Otherwise just use prop as the value.
let value = func ? func(prop) : prop

const x = []
x.push("hi")
console.log(x) // outputs ["hi"]

x = 1 // TypeError: Assignment to constant variable.
const x = 2 // SyntaxError: Identifier 'x' has already been declared

const zeroPad = (num, places) => String(num).padStart(places, '0')

console.log(zeroPad(5, 2)); // "05"
console.log(zeroPad(5, 4)); // "0005"
console.log(zeroPad(5, 6)); // "000005"
console.log(zeroPad(1234, 2)); // "1234"