Validating values

A common task in untyped languages is confirming the value type that you're working with. For example, when you think a variable is a Number,

console.log('$' + price.toFixed(2));

But you get an error:

Uncaught TypeError: price.toFixed is not a function

Maybe the value of price was submitted from a form,which always converts to a String, and does not have the toFixed method. Errors like this can be avoided by:

1. Testing for the correct type and/or;
2. Converting to the expected type explicitly

Here are the generally accepted methods of doing both for each of the common value types.

Undefined?

if (typeof myVar !== 'undefined') {
  // Variable has been assigned!
}

String

Test for String value type:

if (typeof myVar === 'string') {
  // Variable is a string!
}

Number

Test for Number value type:

if (typeof myVar === 'number') {
  // Variable is a number!
}

Boolean

Test for Boolean value type:

if (typeof myVar === 'boolean') {
  // Variable is a boolean!
}

Null

Test if a variable is NOT null:

if (typeof myVar === "object" && !myVar) {
  // Variable is a NOT null!
}

• typeof null evaluates to "object" because of an old and non-fixable bug in Javascript. See How to Check for null in JavaScript for all the gory details.

Array

Test for Array value type:

// Method 1
if (myVar.constructor == Array) {
  // Variable is an array!
}

// Method 2
if (Array.isArray(myVar)) {
  // Variable is an array!
}

• source

Test for a NON-EMPTY array:

// Method 1
if (myVar.constructor == Array && myVar.length) {
  // Variable is anon-empty array!
}

// Method 2
if (Array.isArray(myVar) && myVar.length) {
  // Variable is anon-empty array!
}

Object

Test for Object value type:

if (typeof myVar === 'object' && myVar !== null) {
  // Variable is an object!
}

• null variables also return "object" because of a bug in Javascript, hence the extra test for null

Optional chaining operator (?.):

const myVar = myObject?.name;

• returns undefined (instead of an error) if name does not exist