{%hackmd BJrTq20hE %}

# ES6 CheatSheet

- <code>let</code> allow javascript to return error when variable was called twice, u can rewrite variable through <code>variable = "String"</code>.
- <code>"use strict"</code> is a good debugging tool
- using <code>let</code> in function argument for same variable wont overwrite the value of the variable

Example:

function checkScope() {
"use strict";
  let i = "function scope";
  if (true) {
    let i = "block scope";
    console.log("Block scope i is: ", i);
  }
  console.log("Function scope i is: ", i);
  return i;
}

// return =
// Block scope i is: block scope
// Function scope i is: function scope

- <code>const</code> is read-only, value cant change, it is a good practice to write const variable in **CAPS**.
- <code>const variable = ["array"]</code> the value of array can be change if we change 1 by 1 using array index.

Example:

const s = [5, 7, 2];
function editInPlace() {
  "use strict";

  //s = [2, 5, 7];
  s[0] = 2;
  s[1] = 5;
  s[2] = 7;

}
editInPlace();

- <code>Object.freeze(CONSTANT)</code> allow error to be return on const object.property when object.property was changed

- u can return a function inside a function to pass it to other variable to call the return function, [**More Explaination**](https://stackoverflow.com/questions/7629891/functions-that-return-a-function?fbclid=IwAR3ZgGI71adRjmYakhhuilvas_RD2N7dl6FsTchMas2fE7dUXqwwl2T2mOg), this is called [**closure**](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures)

let counter = () => {
    let count = 0;
    return () => alert(count++);
}

const x = counter();
x();
x();
x();

- ES6 allow u to set a default parameter on return function
- REST function - <code>...</code> can let indefinite parameter to be define as an array

const sum = (function() {
  return function sum(...args) {
  // reduce() executes a function on element in an array,result to single output
  // parameter.reduce((array, var) => argument, var-value)
    return args.reduce((a, b) => a + b, 0);
  };
})(); // () to change function to value to call in future
console.log(sum(1, 2, 3, 4));

### Higher Order Function

### Higher Order Arrow Function Example:

const realNumberArray = [4, 5.6, -9.8, 3.14, 42, 6, 8.34, -2];

const squareList = (arr) => {
// filter an array with an argument
// array.filter(variable => argument)
  const squaredIntegers = arr.filter(num => Number.isInteger(num) && num > 0).map(x => x * x);
  return squaredIntegers;
}

const squaredIntegers = squareList(realNumberArray);
console.log(squaredIntegers);