To concat variables in console.log you can separate them with comma.Like below:

var foo = "bar";
console.log("Hello to",foo);

output : Hello to bar

It will automatically add space betweens variables and strings!

Usin % in strings concatenation is called placeholders.

var foo = "bar";
console.log("Hello to %",foo);

output : Hello to bar

DOM stands for Document Object Model.It is an object oriented representation of HTML or XML.

Shorthand of window.alert is alert.Because window is a global object. Alert works a modal prompt.It meand it doesn't work until the prompt is answered.

It will get an input from user in window alert.Example:

var value = prompt("Give us a value.")

If user doesn't accept to return a value: in safari -> empty string in other browsers -> null value

It will show a ok cancle screen to ask from user to have some behaviour or not.

Term undefined means that the variable is decalred but no value is assinged whereas null is a value for a variable.

By the way undefined is a type and null is an object.

some important globalNumber variables or other number global variables:

Infinity // a number more than any other Number
Number.POSITIVE_INFINITY // a number more than any other Number
-Infinity // a number less than any other Number
Number.NEGATIVE_INFINITY; // a number less than any other Number

shuffle data:

let items = [1,2,3,4];
items.sort((a, b) => Math.random() > .5 ? -1 : 1);
console.log(items); // [2,1,4,3]

JS replace needs to have regex to replace all occurences:

'repetitive'.replace(/e/g, '#')

output: r#p#titive

var o = { 'foo': 'bar' };

console.log('constructor' in o); // TRUE
console.log('foo' in o); // TRUE

console.log(o.hasOwnProperty('constructor')); // FALSE
console.log(o.hasOwnProperty('foo')); // TRUE

push is mutable while concat doesnt change the original array.

let i = [1,2,3]
i.push(4)
console.log(i) // [1,2,3,4]
i.concat(5)
console.log(i) // [1,2,3,4]