Values, types and let/const

JavaScript values are numbers (one type covers whole numbers and decimals), strings, booleans, null, undefined, objects and a few rarer types. Declare names with const by default, and with let when you must reassign them. Both have block scope: they exist only inside the nearest pair of curly braces.

var is the old form with confusing scoping rules, so leave it out of new code. const stops reassignment, not change: you can still push to a const array. JavaScript also converts types silently, which is called type coercion.

So "5" + 1 gives "51" while "5" - 1 gives 4. == converts both sides before comparing, while === (strict equality) does not. null and undefined both mean a missing value, and it pays to check for them.

Use === everywhere, check a value's type with typeof, and convert input on purpose with Number(). These rules explain most of the strange output beginners hit.