JavaScript Data

JavaScript Template Literal

Data

Let’s start off by looking at the building blocks of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript’s types are:

• Number
  • 64 bit double
• BigInt
• String
  • JavaScript String
• Boolean
  • Boolean()
• Symbol (new in ES2015)
  • JavaScript Symbol
• Object
  • Function
  • Array
  • Date
  • RegExp
• null
• undefined

Variable

New variables in JavaScript are declared using one of three keywords: let, const, or var.

Let

let allows you to declare block-level variables. The declared variable is available from the block it is enclosed in.

let a;
let name = 'Simon';

The following is an example of scope with a variable declared with let:

// myLetVariable is *not* visible out here
 
for (let myLetVariable = 0; myLetVariable < 5; myLetVariable++) {
  // myLetVariable is only visible in here
}
 
// myLetVariable is *not* visible out here

Const

const allows you to declare variables whose values are never intended to change. The variable is available from the block it is declared in.

const Pi = 3.14; // variable Pi is set
Pi = 1; // will throw an error because you cannot change a constant variable.

Var

var does not have the restrictions that the other two keywords have. This is because it was traditionally the only way to declare a variable in JavaScript. A variable declared with the var keyword is available from the function it is declared in.

var a;
var name = 'Simon';

An example of scope with a variable declared with var:

// myVarVariable *is* visible out here
 
for (var myVarVariable = 0; myVarVariable < 5; myVarVariable++) {
  // myVarVariable is visible to the whole function
}
 
// myVarVariable *is* visible out here

If you declare a variable without assigning any value to it, its type is undefined.
An important difference between JavaScript and other languages like Java is that in JavaScript, blocks do not have scope; only functions have a scope. So if a variable is defined using var in a compound statement (for example inside an if control structure), it will be visible to the entire function. However, starting with ECMAScript 2015, let and const declarations allow you to create block-scoped variables.

Difference Between let and var

javascript - What is the difference between “let” and “var”? - Stack Overflow

• let is visible to the scope
• var is visible to the function, and it will create a property on the global object

Hacks

• Use + prefix to convert something to number