3 Basic JavaScript Concepts That Are Important.

Learning javascript is weird. I want to share 3 things that I learnt today in javascript.

Hoisting

Hoisting is a mechanism where variable or function declarations are moved to the top of their scope before they're executed. That means, if we do this:

console.log(greeter);
var greeter = "say Hello!";

It'll be interpreted as this:

var greeter;
console.log(greeter); //undefined
greeter = "say Hello!";

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null vs undefined

null is a value, just like 2, 3.5 or true. Setting a variable to null means that you have declared a variable and decided to give the value - nothing, to it.
undefined on the other hand, means you have declared a variable but not given it any value yet.

Interestingly, null loosely equalizes to undefined but there are a few gotchas!

console.log(null == undefined); //true
console.log(null === undefined); //false
console.log(null >= 0); //true
console.log(null <= 0); //true
console.log(undefined <= 0); //false
console.log(undefined >= 0); //false

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== vs ===

This is a simple one. == means loose equality. It's used when you want to know if two values are equal if they'd have same type. For example:

console.log(0 == false); //true
console.log(1 == 1.0); //true
console.log(1 == '1'); //true

=== however, checks for strict equality. It means, it'll only return true IF the two values are absolutely same. For example:

console.log(0 === false); //false
console.log(1 === 1.0); //true, it's a bad idea to compare floats anyway.
console.log(1 === '1'); //false

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That's all folks!

Comments

[Anirudh Madhavan]

Hoisting can be done only on variables defined using 'var' and function definitions excluding function expressions. In the above two cases , variables are partially hoisted (undefined) and functions are fully hoisted.

[Manav]

Ah. Thanks for that new information!