Note_Tech

All technological notes.

---

Project maintained by simonangel-fong Hosted on GitHub Pages — Theme by mattgraham

JavaScript - Data Types

Back

• JavaScript - Data Types
  • Data Types
  • JavaScript Primitives
  • typeof Operator
    • Strings
    • Numbers
    • BigInt
    • Boolean
    • Arrays
    • Objects
    • Undefined

---

Data Types

JavaScript Primitives

• primitive value

  • a value that has no properties or methods.

  • 3.14 is a primitive value.

  • Immutable
    • Primitive values are immutable (they are hardcoded and cannot be changed).
    • if x = 3.14, you can change the value of x, but you cannot change the value of 3.14.

• primitive data type

  • data that has a primitive value.

• JavaScript defines 7 types of primitive data types:

  1. String
  2. Number
  3. Bigint
  4. Boolean
  5. Undefined
  6. Null
  7. Symbol

• Object contain values

  1. An object
  2. An array
  3. A date

• A JavaScript variable can hold any type of data.

  // Numbers:
  let length = 16;
  let weight = 7.5;
  
  // Strings:
  let color = "Yellow";
  let lastName = "Johnson";
  
  // Booleans
  let x = true;
  let y = false;
  
  // Object:
  const person = { firstName: "John", lastName: "Doe" };
  
  // Array object:
  const cars = ["Saab", "Volvo", "BMW"];
  
  // Date object:
  const date = new Date("2022-03-25");
  

• When adding a number and a string, JavaScript will treat the number as a string.

  • JavaScript evaluates expressions from left to right. Different sequences can produce different results

  let x = 16 + "Volvo"; //16Volvo
  
  let x = "Volvo" + 16; //Volvo16
  
  let x = 16 + 4 + "Volvo"; // 20Volvo
  
  let x = "Volvo" + 16 + 4; // Volvo164
  

• JavaScript Types are Dynamic

  • JavaScript has dynamic types. This means that the same variable can be used to hold different data types.

  let x; // Now x is undefined
  x = 5; // Now x is a Number
  x = "John"; // Now x is a String
  

---

typeof Operator

• The typeof operator returns the type of a variable or an expression.

typeof ""; // Returns "string"
typeof "John"; // Returns "string"
typeof "John Doe"; // Returns "string"
typeof 0; // Returns "number"
typeof 314; // Returns "number"
typeof 3.14; // Returns "number"
typeof 3; // Returns "number"
typeof (3 + 4); // Returns "number"

---

Strings

• A string (or a text string) is a series of characters.

• Strings are written with quotes. You can use single or double quotes.

• You can use quotes inside a string, as long as they don’t match the quotes surrounding the string.

// Single quote inside double quotes:
let answer1 = "It's alright";

// Single quotes inside double quotes:
let answer2 = "He is called 'Johnny'";

// Double quotes inside single quotes:
let answer3 = 'He is called "Johnny"';

---

Numbers

• All JavaScript numbers are stored as decimal numbers (floating point).

• Numbers can be written with, or without decimals

• Exponential Notation

  • Extra large or extra small numbers can be written with scientific (exponential) notation.

  let y = 123e5; // 12300000
  let z = 123e-5; // 0.00123
  

---

BigInt

• All JavaScript numbers are stored in a a 64-bit floating-point format.

• JavaScript BigInt is a new datatype (2020) that can be used to store integer values that are too big to be represented by a normal JavaScript Number.

---

Boolean

• Booleans can only have two values: true or false.

• Booleans are often used in conditional testing.

---

Arrays

• JavaScript arrays are written with square brackets.

• Array items are separated by commas.

• Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

const cars = ["Saab", "Volvo", "BMW"];

---

Objects

• JavaScript objects are written with curly braces {}.

• Object properties are written as name:value pairs, separated by commas.

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 50,
  eyeColor: "blue",
};

---

Undefined

• In JavaScript, a variable without a value, has the value undefined. The type is also undefined.
• Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

let car; // Value is undefined, type is undefined

let car = "Volvo";
car = undefined; //undefined
typeof car; //undefined

---

TOP