JavaScript Data Types

In JavaScript, every value is either an object or a primitive value. A value is only a primitive when it is not an object.

Note

JavaScript is dynamically typed, so we do not need to manually define the data type stored in a variable; this is done automatically.

Note

In JavaScript, the value has a type, not the variable

Primitive Data Types

• String → defined with single or double quotes or backticks (for use as f-string)
• Numbers → as in Python (note that all numbers are floating point numbers)
• Boolean → true/false
• null → null
• undefined → undefined
• Symbol (not very common)
• BigInt → Larger integers that number cannot hold (like smallInt, int, and bigint in SQL)

We can test types with `console.log(typeof ).

const name = "John"
 
console.log(typeof name);-> string
 
console.log(typeof 30.5); -> number
 
console.log(typeof null); -> "object"
 
console.log(typeof undefined);-> undefined

Info

typeof null returns “object” due to an early implementation of JavaScript, wherein objects had tags and values, and null was given a value of 0

Strings

String concatenation can be used by operator overloading or with template literals. The latter is like Python f-strings, except that it requires the use of back-ticks (`) when writing the string.

// Operator overloading
const name = "John";
const age = 30;
 
console.log("My name is " + name + " and I am " + age);

// Template String (like Python f-string)
console.log(`My name is ${name} and I am ${age}`)

We can also use template literals to create multi-line strings. Without the template literals we can to use the /n character plus a termination \ for each line:

console.log("String with \n\
multiple \n\
lines")

With a template literal we can do:

console.log(`String with
multiple
lines`)

Note

The use of backticks for template literals is mandatory. However, it is also possible to default to always using backticks for all strings.

Arrays

Arrays are much like Python lists. We initialize an array either via a constructor, or by using Python list-like syntax:

const numbers = new Array(1,2,3,4,5);
console.log(numbers);
 
  
const fruits = ["apples", "bananas", "oranges"]
console.log(fruits[1]); // Prints "bananas"
fruits[3] = "grapes";
fruits.push("mangoes") // Adds value to end of array
fruits.unshift("lemons") // Adds value to beginning of array
fruits.pop() // Remove last item
fruits.indexOf("bananas")

Note

In the example above I used const to declare the fruits array. I then modified the array, which const allows us to do. We just can’t re-declare it.