Data types and arrays

Every value in JavaScript has a type. Numbers such as 100 and 36.5 are number. Text inside quotes such as "Hello" is a string. The yes/no values true and false are boolean. The word typeof tells you the type of a value:

console.log(typeof 500);
console.log(typeof "500 coins");
console.log(typeof "100");
console.log(typeof true);
console.log(typeof (3 > 5));

number
string
string
boolean
boolean

The type changes the result: + adds two numbers, but with a string it glues them together. That is why we convert with Number() and String(). Careful: "100" in quotes is a string, not a number, and Number("Hello") gives NaN, "Not a Number":

console.log("500" + 5);
console.log(Number("500") + 5);
console.log(String(500) + 5);
console.log(Number("Hello"));

5005
505
5005
NaN

Arrays

To store a week of temperatures you would need seven variables. It is easier to put them all in one variable called an array: values inside square brackets [ ], separated by commas. Each value has a position number (index) that starts at 0; you write it in square brackets after the array name.

let tempList = [36, 36, 26, 27, 28, 33, 36];
console.log(tempList);
console.log(tempList[0]);
console.log(tempList[4]);

[36, 36, 26, 27, 28, 33, 36]
36
28

tempList.length gives the number of values in the array. Since counting starts at 0, the last value is at length - 1. Asking for a position that does not exist gives undefined:

let tempList = [36, 36, 26, 27, 28, 33, 36];
console.log(tempList.length);
console.log(tempList[tempList.length - 1]);
console.log(tempList[7]);

7
36
undefined

Changing a value

Change a value inside an array by assigning to its position, just like any variable. Arrays can hold strings too; when printed, the strings appear in quotes:

let marks = [70, 85, 90];
marks[1] = 95;
console.log(marks);

let names = ["Mona", "Omar", "Sara"];
names[0] = "Laila";
console.log(names);

[70, 95, 90]
["Laila", "Omar", "Sara"]