JavaScript variables

Variables store data for later use:

Statements

JavaScript statements take on a form similar to a mathematical equation (a variable gets a value)...

...but in a manner similar to an assignment (see the second line):

let x = 2 
x = x + 1
// 3

In the example above, we do not mean that x is equal to x+1 which makes no sense but that:

"x will have a value ... of the last known value of x, which is 2, and then added 1, to make 3!"

Another way of looking at this is by thinking of the single = sign as an left-facing arrow:

x <- 2
x <- x + 1

"let x be 2, then let x be 2 + 1"

Semi-colons at the end

In ES6, semi-colons (;) have become optional

• some programmers prefer to continue using them out of habit

• others just want to keep their a little cleaner!

Declarations

Indeed, we use the reserved keywords const and let to declare what kind of statement that we wish to make:

• const is a variable but up until the point we declare it

  • we can change its value at its line of declaration

  • ...but we cannot re-assign it at any other point in the program

• let is a variable in its truest form

  • we can change its value after its declaration

// we can change "100" here...
const x = 100 

// ...but we cannot change it here
x = 200 

// we can change "100" here...
let y = 100 

// ...and we can change it here because of let
y = 200

Reserved keywords

When we declare variables, we cannot use some reserved keywords as variable names, e.g.:

let let = 1 // no
let const = 1 // no
const let = 1 // no
const const = 1 // no

...won't work because the language won't let us use them as variable names!

Keyword var

var is a variable like let but:

• let has block scope

  • it exists only within the confines of its closest curly braces ({})

• var has function scope

  • it exists within the confines of its function

function doSomething() {
// function scope for var

    const x = 10

    if (x > 0) {
    // block scope for let
        let z = 100
        z = 200    // we can change it here
        var t = 1
    }

    // "1" (this call exists inside the function scope)
    console.log(t) 

    // undefined (this call exists outside the block scope)
    console.log(z) 

}