JavaScript is dynamically typed. You don't need to tell the engine what type your variable is.

6 Primitive types:

• undefined - represents lack of existence
• null - represents lack of existence (Use this to set variable only)
• Boolean
• Number - A floating point number really
• String
• Symbol - New primitive in ES6

Object and Array are reference types.

// Right-to-left Associativity
a = b = c;

Converting a value from one type to another.

Since JavaScript is dynamically typed, coercion happen a lot.

3 < 2 < 1; // Warning: Weird part
false < 1;
Number(false) < 1

Double equality will do coercion. Strict equality will not!

Object.is is even better than strict equality.

Equality comparisons and sameness

Boolean(undefined); // false
Boolean(null);      // false
Boolean("");        // false
Boolean(0);         // Warning: Also false

// Thus if you want to check for undefined, null or empty string
// you can safely use double equality for coercion.
// But beware of ZERO.

typeof [] // object
Object.prototype.toString.call([]) // [object Array]

john instanceof Person

typeof undefined // ok
typeof null // Bug! object!!!!!