minte9

LearnRemember

Variables

 
/**
 * Local variables are visible only inside a function
 * A function has full access to an outer variable
 * 
 * Modern code has few or no global variables
 */

let out = 'World';

function hello() {
    let msg = 'Hello ' + out;
    out = 'Javascript';
}

hello();

console.log(typeof(msg) == "undefined"); // true
console.log(typeof(out) == "undefined"); // false

Return

 
/**
 * It is posible to use return without value ...
 * which causes the function to exit (returns undefined)
 * 
 * Functions used very often can have ultrashort names,
 * but these are exceptions:  $() jQuery, _() Lodash
 */

function check(age) {

    if (!age)       return; // Look Here
    if (age > 18)   return 'OK';
}

console.log(check());   // undefined
console.log(check(20)); // OK

Readability

 
/**
 * A separate function act as a great comment
 * It good to create functions even if we don't reuse them
 */

function showPrimes(n) {
    for (let i=2; i<n; i++) {
        if (isPrime(i)) {
            console.log(i);
        }
    }
}

function isPrime(n) {
    for (let i=2; i<n; i++) {
        if (n%i == 0) return false;
    }
    return true;
}

showPrimes(10); // 2 3 5 7

Expression

 
/**
 * The common syntax to create a function ...
 * is called function declaration
 * 
 * In Javascript, a function is just another type of value
 * So, we cand use function expression
 * 
 * Function Expression are created when the exection reaches them
 * Function Declaration it's created on initialization stage
 */

let sayHi = function() { // Look Here
    console.log('Hello World');
};

console.log(typeof(sayHi)); // function

sayHi(); // Hello World

Arrow

 
/**
 * Arrow function
 * 
 * It is a shorter form for function expression
 * 
 * Only one parameter, ...
 * the parentheses can be omitted
 * 
 * No parameter, ...
 * the parentheses should be present
 */

let sum = (a, b) => a + b;
let double = x => x*2;
let pi = () => Math.PI.toFixed(2);

console.assert(sum(1,2) == 3);  // pass
console.assert(double(3) == 6); // pass
console.assert(pi() == 3.14);   // pass