You can't reassign constants.

const foo = "bar";
foo = "hello";

Prints:

Uncaught TypeError: Assignment to constant.

Declaring a variable const only prevents its value from being replaced by a new value. const does not put any restrictions on the internal state of an object. The following example shows that a value of a property of a const object can be changed, and even new properties can be added, because the object that is assigned to person is modified, but not replaced.

const person = { 
    name: "John" 
};
console.log('The name of the person is', person.name);

person.name = "Steve";
console.log('The name of the person is', person.name);

person.surname = "Fox";
console.log('The name of the person is', person.name, 'and the surname is', person.surname);

Result:

The name of the person is John
The name of the person is Steve
The name of the person is Steve and the surname is Fox

In this example we've created constant object called person and we've reassigned person.name property and created new person.surname property.

You can initialize a constant by using the const keyword.

const foo = 100;
const bar = false;
const person = { name: "John" };
const fun = function () = { /* ... */ };
const arrowFun = () => /* ... */ ;

Important
You must declare and initialize a constant in the same statement.

There are four principle ways to declare a variable in JavaScript: using the var, let or const keywords, or without a keyword at all ("bare" declaration). The method used determines the resulting scope of the variable, or reassignability in the case of const.

• The var keyword creates a function-scope variable.
• The let keyword creates a block-scope variable.
• The const keyword creates a block-scope variable that cannot be reassigned.
• A bare declaration creates a global variable.

var a = 'foo';    // Function-scope
let b = 'foo';    // Block-scope
const c = 'foo';  // Block-scope & immutable reference

Keep in mind that you can't declare constants without initializing them at the same time.

const foo; // "Uncaught SyntaxError: Missing initializer in const declaration"

(An example of keyword-less variable declaration is not included above for technical reasons. Continue reading to see an example.)

JavaScript variables can hold many data types: numbers, strings, arrays, objects and more:

// Number
var length = 16;

// String
var message = "Hello, World!"; 

// Array
var carNames = ['Chevrolet', 'Nissan', 'BMW']; 

// Object
var person = {
    firstName: "John",
    lastName: "Doe"
}; 

JavaScript has dynamic types. This means that the same variable can be used as different types:

var a;              // a is undefined
var a = 5;          // a is a Number
var a = "John";     // a is a String

Declared variable without a value will have the value undefined

var a;

console.log(a); // logs: undefined

Trying to retrieve the value of undeclared variables results in a ReferenceError. However, both the type of undeclared and unitialized variables is "undefined":

var a;
console.log(typeof a === "undefined"); // logs: true
console.log(typeof variableDoesNotExist === "undefined"); // logs: true

To assign a value to a previously declared variable, use the assignment operator, =:

a = 6;
b = "Foo";

As an alternative to independent declaration and assignment, it is possible to perform both steps in one statement:

var a = 6;
let b = "Foo";

It is in this syntax that global variables may be declared without a keyword; if one were to declare a bare variable without an assignment immediately afterword, the interpreter would not be able to differentiate global declarations a; from references to variables a;.

c = 5;
c = "Now the value is a String.";
myNewGlobal;    // ReferenceError

Note, however, that the above syntax is generally discouraged and is not strict-mode compliant. This is to avoid the scenario in which a programmer inadvertently drops a let or var keyword from their statement, accidentally creating a variable in the global namespace without realizing it. This can pollute the global namespace and conflict with libraries and the proper functioning of a script. Therefore global variables should be declared and initialized using the var keyword in the context of the window object, instead, so that the intent is explicitly stated.

Additionally, variables may be declared several at a time by separating each declaration (and optional value assignment) with a comma. Using this syntax, the var and let keywords need only be used once at the beginning of each statement.

globalA = "1", globalB = "2";
let x, y = 5;
var person = 'John Doe',
    foo,
    age = 14,
    date = new Date(); 

Notice in the preceding code snippet that the order in which declaration and assignment expressions occur (var a, b, c = 2, d;) does not matter. You may freely intermix the two.

Function declaration effectively creates variables, as well.

Increment by

var a = 9,  
b = 3;  
b += a;  

b will now be 12

This is functionally the same as

b = b + a; 

Decrement by

var a = 9,  
b = 3;  
b -= a;  

b will now be 6

This is functionally the same as

b = b - a;  

Multiply by

var a = 5,  
b = 3;  
b *= a;  

b will now be 15

This is functionally the same as

b = b * a;  

Divide by

var a = 3,  
b = 15;  
b /= a;  

b will now be 5

This is functionally the same as

b = b / a;  

var a = 3,  
b = 15;  
b **= a;  

b = b ** a;