Checking Types in JavaScript #javascript

check_types.js

// you often want to check if a value is truthy
function booleanise(v) {
  return !!v;
}

booleanise(0) // false
booleanise('') // false (empty strings is falsey)
booleanise(false) // false
booleanise(null) // false
booleanise(undefined) // false
boolanise([]) // true (empty arrays are truthy!)

// sometimes you want to check if a value is null or undefined only
function nully (v) {
  return v == null;
}

nully(null) // true
nully(undefined) // true
nully(false) // false

// so you can use if (v != null) { ... } to prevent nully values

// to check if a value is other primitive types use typeof
// for arrays use Array.isArray
// for normal objects use `value && typeof value === 'object' && value.constructor === Object`
// for classes use instanceof
// for functions use typeof