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Data Types

JavaScript is a weakly (loosely) typed language as opposed to Swift that is a strongly typed language.

Example of a weakly typed language

In this example the type name is not strongly tied to forever be a String. Re-assigning the data type to be a Number does not cause a compile-time error like a language like Swift. This causes the developer to check types when working through various coding logic.

let name = "Alex"; // name here is a string
name = 10; // name is now a number

console.log(`${name + 7}`); // name here is a number

Example of a strongly typed language

In this Swift example we would get a compile time error as we cannot re-assign the name data type to an Int.

var name = "Alex";
name = 10; // COMPILER ERROR - cannot assign type Int to expected type String

print("\(name + 7)") // COMPILER ERROR - cannot convert value Int to expected value String

Simple Types

  • Number (float)
  • String
  • Boolean
  • Date
  • Function
  • Array
  • Object

Special Types

  • NaN
  • null
  • undefined

Checking Type

typeof

Returns a string of the data type primitive.

typeof('hello'); // string 
typeof(2); // number 
typeof(true); // boolean

instanceof

Returns a Boolean of if a value matches the data type.

let str = new String() // new instance of String

if (str instanceof String) {
  console.log('It is a String value') // It is a String value
}

More examples of testing typeof and instanceof

const fellows = ['Brian', 'Herb', 'Ashley', 'Kim']; 
const isFriday = true; 
const year = 2020;
const description = 'Incredible'; 
const person = {
  'firstName': 'Alex',
  'lastName': 'Paul'
};

function greeting(aPerson) {
  console.log(`The person\'s first name is ${aPerson.firstName}`);
}

console.log('----typeof----');
console.log(typeof fellows); // object
console.log(typeof isFriday); // boolean
console.log(typeof year); // number
console.log(typeof description); // string
console.log(typeof person); // object
console.log(typeof greeting); // function

console.log('----instanceof----');
console.log(fellows instanceof Array); // true 
console.log(isFriday instanceof Boolean); // false 
console.log(year instanceof Number); // false 
console.log(description instanceof String); // false 
console.log(person instanceof Object); // true
console.log(greeting instanceof Function); // true

Note: console.log(isFriday instanceof Boolean) returns false since this is a literal declaration and NOT a constructor instance.

A constructor instance would be

let newStringInstance = new String('Hello'); 

console.log(newStringInstance instanceof String); // true

Using typeof check in a function

function add(a, b) {
  if ((typeof a === 'number') && (typeof b === 'number')) {
    return a + b; 
  }
  return -1; 
}

console.log(add('alex', 5)); // -1
console.log(add(10, 5)); // 15

Equality

let x = 0 == ''; // true, type coerced (evaluates as the same type) 

let x = 0 === ''; // false, better usage for equality as it respects the types in the case of `===`

Reminder: Always use the === to test types for equality.

Challenges

Challenge 1

Fill out the correct data type next to the comments in the code below

const length = 16;            // 
const lastName = 'Johnson';   // 
const x = {
  firstName: 'John',
  lastName: 'Doe'
};                            //
Solution 
const length = 16;            // number
const lastName = 'Johnson';   // string
const x = {
  firstName: 'John',
  lastName: 'Doe'
};                            // object

Challenge 2

Complete the following data type predictions.

typeof(15)
// Prediction:
// Actual:

typeof(5.5)
// Prediction:
// Actual:

typeof(NaN)
// Prediction:
// Actual:

typeof("hello")
// Prediction:
// Actual:

typeof(true)
// Prediction:
// Actual:

typeof(1 != 2)
// Prediction:
// Actual:
Solution 
typeof(15)
// Prediction: number
// Actual: number

typeof(5.5)
// Prediction: number
// Actual: number

typeof(NaN)
// Prediction: number
// Actual: number

typeof("hello")
// Prediction: string
// Actual: string

typeof(true)
// Prediction: boolean
// Actual: boolean

typeof(1 != 2)
// Prediction: boolean
// Actual: boolean