JavaScript Basics 1

Introduction

A review of the basics of JavaScript.

Although ECMAScript 6 (ES6) is the latest standard, adopted in June of 2015, we'll focus on features from the ES5 standard. Some of the references in this document may include descriptions of ES6 features. These features will usually be denoted with (new in ECMAScript 6) in the main text or with a flask icon in the navigation sidebar.

Objectives

By the end of this lesson, students should be able to:

List all 5 JavaScript primitives and give an example of each
Identify the operator in an expression and explain what it does
Define variable and contrast with value
Evaluate simple JavaScript by inspection
Write simple scripts that use flow control

To start

In your browser open https://github.com/ga-wdi-boston/js-fundamentals-08 and fork the repository. We'll find the link in slack.

Then in a terminal window at the shell prompt:

~$ cd wdi
~/wdi$ git clone [email protected]:<github user name>/js-basics-1.git
~/wdi$ cd js-basics-1
~/wdi/js-basics-1$ git branch lesson
~/wdi/js-basics-1$ subl .
~/wdi/js-basics-1$ npm install

Basics

Node.js

We'll use Node.js as a REPL and a script runner to explore JavaScript features.

Read
Evaluate
Print
Loop

~/wdi/js-basics-1$ node
>

What other tools could we use as a JavaScript REPL?

Are there benefits or drawbacks to using one over another?

What about a script runner?

Benefits or drawbacks with the options available?

Primitive types

ES5 has 5 primitive types: Number, String, Boolean, null, and undefined.

Type	Examples
Number	`-0`, `NaN`, `Infinity`
String	`''`, `"The non-empty string."`
Boolean	`true`, `false`
null	`null`
undefined	`undefined`

Primitive types represent immutable values. We'll contrast this with reference types in a later lesson.

The types Number and String both have large sets of possible values. Boolean has only two values and null and undefined each have just one.

Literals

Literals represent specific values in the source code. Some examples are 1, 'A string', null.

Variables

> dob;

Variables need to be declared.

> var dob;

Variables name storage for the value they contain. Because JavaScript is a dynamically typed language, you can assign a value of one type to a variable and then later assign a value of a different type to that same variable.

In JavaScript, null represents the explicitly omitted value, whereas undefined represents the default omitted value. Variables that have been declared but are uninitialized or unset have the value undefined.

Operators

Operators come in three classes, unary, binary (the most common), and ternary (there is only one).

Operator precedence determines the order in which operators are evaluated. Operators with higher precedence are evaluated first.

Associativity determines the order in which operators of the same precedence are processed.

The following table lists a subset of the JavaScript operators from higher to lower precedence.

Type	Associativity	Operators
grouping	n/a	`()`
postfix increment	n/a	`++ --`
negation, numeric conversion, prefix increment, type	right-to-left	`! - + ++ -- typeof`
multiplication, division	left-to-right	`* / %`
addition, subtraction	left-to-right	`+ -`
relation	left-to-right	`< <= > >=`
strict equality	left-to-right	`=== !===`
logical and	left-to-right	`&&`
logical or	left-to-right	`\|\|`
conditional	right-to-left	`?:`
assignment	right-to-left	`= += -= *= /= %= \|=`

Expressions

An expression is a combination of literals, variables, operators, function invocations and sub-expressions that are interpreted and produce a value. Not all such combinations produce sensible results.

The simplest expression is a variable or literal followed by a semicolon. More complicated expressions are formed by combining simpler expressions using operators.

An expression with all of the variables replaced with literals that are equal to the values of the variables will produce the same result.

Numeric

5 + 3;
7 - 2;
11 % 5;

String

var firstName;
var lastName;
var fullName;
firstname = "Antony";
lastName = "Donovan";
fullName = firstName + " " + lastName;

Assignment

var height;
height = 77;
var name;
name = "Antony";
height - 1;
height;
height = height - 1;
height;

Remember: JavaScript variables are untyped.

var height;
height = 76;
height = "Antony";

Although it doesn't cause an error, avoid confusing code like the above.

Now let's compare some common methods of counting.

var i;
i = 0;
i = i + 1;
i = 0;
i += 1;
++i;
i++;
i;

Type conversion

The unary + operator attempts to convert its operand to a Number. If unsuccessful the result is NaN.

If either operand of the binary + operator is a string the operator converts the other operator to a string. Some results of this conversion are more useful than others.

Note the different between 3 + 5 + " times"; and "times " + 3 + 5;?

The unary ! operator converts its operand to a boolean value.

Boolean

A boolean expression is a comparison (e.g. >, >=, ===) or any value interpreted as a boolean. We'll use that fact when we get to flow control. Boolean expression combine using the logical and && and logical || operators.

var height = 62;
height === 60;
height > 72;
height = 76;
height > 72;
height > 72 && height < 78;

The logical operators "short circuit", which means they stop evaluating operands as soon as the expression is false for &&, or true for ||.

truthy and falsy

What do you think of when you here "truthy" and "falsy"?

The falsy list (everything else in JavaScript is truthy),

false
undefined
null
0 // and -0
NaN
"" // and '' - the empty string

Note: The negation of a truthy value is false and the negation of a falsy value is true.

var truthy;
var falsy;
truthy = "A non-empty string";
falsy = 0;
!truthy;
!falsy;

Flow Control

The if statement:

//We'll learn about require later in the course
var ask = require('./ask.js');

var name = ask("What's your name? ");
if (name === 'Antony') {
  console.log('Hi, Antony!');
} else if (name === 'Matt') {
  console.log('Hi, Matt!');
} else {
  console.log('Hi, stranger.');
}

The while loop:

//We'll learn about require later in the course
var ask = require('./ask.js');

var count = 0;
var answer = '';

while (answer !== 'Antony') {
  answer = ask("Guess my name? ");
  count = count + 1;
}
console.log("You got it in " + count + " tries!");

The for loop:

for (var i = 0; i < 10; i++) {
  console.log(i);
}

which is equivalent to:

var i = 0;
while (i < 10) {
  console.log(i);
  i++;
}

Nesting conditionals in loops:

for (var i = 0; i < 10; i++) {
  if (i===5) {
    console.log("five!");
  }
}

Assessment

Please follow the instructions at https://github.com/ga-wdi-boston/js-basics-1-assessment

Additional Resources

See the following sections at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide

Grammar and types
Control flow and error handling
Loops and iteration
Expressions and operators
Number and dates
Text formatting

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