modular-js-notes/js-basics

Intro to JS

Lesson Objectives

1. Add comments to code
2. Describe the basic data types of JS
3. Assign values to variables
4. Concatenate values
5. Boolean expressions
6. Operators
7. Write a While loop
8. Write a For loop

Add comments to code

single line comment:

// this is a single line comment

multi-line comment:

/*
this is
a mult-line
comment
 */

Describe the basic data types of JS

Strings:

console.log("hello world");

Numbers:

console.log(100);

Assign values to variables

var phrase = 'In my room is a chair and a table';
console.log(phrase);

We will not be using var (it's a bit out of date), instead we will be using let and const.

Let's use const here:

const phrase = 'In my room is a chair and a table';
const sum = 99 + 1;

const variables are constant and do not change. Now that the phrase is saved to a variable, we can call it whenever.

console.log(phrase);
console.log(sum);

Variable re-assignment

With const, you cannot reassign a variable. It is CONSTANT.

const item = ' chair';
item = 'eclair';

You can re-assign variables with let:

let item = 'chair';
item = 'eclair';
console.log(item);

Concatenate values

JavaScript allows us to add strings together with +:

console.log('hello' + ' world');

We can insert values of variables into our strings:

const adjective = 'beautiful';
console.log('What a ' + adjective + ' day!!');

Boolean expressions

Test various kinds of equality

• > greater than
• < less than
• == equal to
• >= greater than or equal to
• <= less than or equal to
• != is not equal

console.log(5 <= 10);
console.log("asdf" != "fun");

Operators

Math

We can do arithmetic with numbers:

console.log(100 + 100);
console.log(100 - 50);
console.log(100 * 10);
console.log(100 / 10);

Postfix operator

i++;
i--;

is the same as

i = i + 1;
i = i - 1;

Compound assignment operator +=

i += 3; //increments by 3
i -= 3; //decrement by 3

is the same as

i = i + 3;
i = i - 3;

Write a While loop

let num = 1;

while (num <= 1000) {
	console.log('The number is: ' + num);
	num++;
}

Write a For loop

for (let i=1; i <= 1000; i++) {
	console.log('The number is: ' + i);
}

There are three parts to the 'control panel' of the loop, delimited by the semicolon:

for (initial condition; while condition; repeating expression) {

}

1. some initial code supplied to the loop -- usually a numerical starting value of the loop
2. the condition under which the loop runs -- it will run while the expression is true
3. a repeating expression that runs at the end of each loop -- usually an instruction to increase the numerical starting value