JS Basics

JS variable scoping

1. var just puts it to global of the immediate function its defined in
2. let is just in the bracket that it is defined in
3. const doesn’t change and just acts

Scoping rules

The main difference is scoping rules. Variables declared by var keyword are scoped to the immediate function body (hence the function scope) while let variables are scoped to the immediate enclosing block denoted by { } (hence the block scope).

The reason why let keyword was introduced to the language was function scope is confusing and was one of the main sources of bugs in JavaScript.

Take a look at this example from another Stack Overflow question:

var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
  // and store them in funcs
  funcs[i] = function() {
    // each should log its value.
    console.log("My value: " + i);
  };
}
for (var j = 0; j < 3; j++) {
  // and now let's run each one to see
  funcs[j]();
}

My value: 3 was output to console each time funcs[j](); was invoked since anonymous functions were bound to the same variable.

People had to create immediately invoked functions to capture correct values from the loops but that was also hairy.

Hoisting

Variables declared with var keyword are hoisted and initialized which means they are accessible in their enclosing scope even before they are declared, however their value is undefined before the declaration statement is reached:

let variables are hoisted but not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError. The variable is said to be in the temporal dead zone from the start of the block until the declaration statement is processed.

Promises, async

async keyword makes the function always return a promise. you chain the async function with .then to get the actual value await keyword also does the same thing as .then does.

Promises have 3 states:

1. pending
2. resolved
3. rejected