VARIABLES IN JAVASCRIPT
Variable is used to label and store data in computer memory.
Variable can update information stored in variable, git reference or get information stored in a variable.
It's important to distinguish that variable is not a value, they contain value and represent them.
let and const are new keyword to declare a variable, introduced in ES6. Before that programmers can declare a variable with only the var keyword. example:
var food = 'pizza';
console.log(food); // output: pizza
let keyword signal that values can be reassigned a different value e.g
let myName = 'Azeez';
console.log(myName); //output: Azeez
myName = 'Rotimi';
console.log(myName); //output: Rotimi
var also can be reassigned in the same way.
var and let are both used for variable declaration in javascript but the difference between them is that var is function scoped and let is block scoped. example:
console.log(x);
var x=5;
console.log(x); //Output: undefined, 5
if an undeclared variable is logged to the console and later declared with var and assigned a value, it will return undefine and the value assigned to it.
an error will be returned if let is used in declaring such variable. example:
console.log(x); // output: ReferenceError: x is not defined
let x=5;
console.log(x); //Output: 5
If the let variable is declared without a value, the variable will be automatically initialized with a value of undefined example:
let myAge;
console.log(myAge); // output: undefined.
The const keyword however cannot because its constant. If you try reassigning a const variable, a TypeError, will be thrown. example:
const myCity = "Lagos";
console.log(myCity); // output: Lagos
myCity = "Ibadan";
console.log(myCity); // output: TypeError
If a const variable is not assigned any value, a SyntaxErrorwould be thrown. example:
const myAge; // output: SyntaxError
const keyword should only be used if programmers are sure it won't be reassigned or redeclared in the future.
Mathematical Assignment Operators:
This is using math operators and variable to calculate new variable and assign to a variable. example:
let goal = 2;
goal = goal + 3;
console.log(goal); // output: 5
In the above example, we declared the variable goal with the number 2 and assigning to it goal = goal + 3 increased the value of goal = 2, to 5.
goal can be reassigned by using built-in mathematical assignment operator. example:
let goal = 2;
goal += 3;
console.log(goal); // output: 5
same can be done for other math operator
-, *, /, %
The Increment and Decrement Operators.
Increment operator ++ and Decrement operator -- are other mathematical assignment operators.
The Increment ++ increase the value by 1. example:
let goal = 2;
goal ++;
console.log(goal); // output: 3
The Decrement ++ decrease the value by 1. example:
let goal = 2;
goal --;
console.log(goal); // output: 1
String Concatenation with Variable
The + operator can used to combine two string values even if those value are being stored in variable. example:
let song = 'indigo';
console.log('I love ' + song + '.'); // I love indigo.
String Interpolation
With E6 we can insert, or interpolate, variable into string using template. example:
const myPet = 'cat';
console.log(`I own a pet ${myPet}.`);
// Output: I own a pet cat.
- A template literal is wrapped by backticks (`).
- Inside the template literal, you’ll see a placeholder, ${myPet}. The value of myPet is inserted into the template literal.
- When we interpolate
I own a pet ${myPet}., the output will print is the string: 'I own a pet cat.'
Typeof
The typeof operator checks the value to its right and returns or passes back, a string of the data type.
const food = 'Beans';
console.log(typeof food); // Output: string
const total = 10;
console.log(typeof total); // Output: number
const string = false;
console.log(typeof string); // Output: boolean
other typeof include undefined, null, symbol, object.
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