3
Language Basics

WHAT'S IN THIS CHAPTER?

• Reviewing syntax
• Working with data types
• Working with flow-control statements
• Understanding functions

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Please note that all the code examples for this chapter are available as a part of this chapter's code download on the book's website at www.wrox.com/go/projavascript4e on the Download Code tab.

At the core of any language is a description of how it should work at the most basic level. This description typically defines syntax, operators, data types, and built-in functionality upon which complex solutions can be built. As previously mentioned, ECMA-262 defines all of this information for JavaScript in the form of a pseudolanguage called ECMAScript.

ECMAScript as defined in ECMA-262, fifth edition, is the most-implemented version among web browsers. The sixth edition is the next to be implemented in browsers, and as of the end of 2017, most major browsers have mostly or fully implemented...

SYNTAX

ECMAScript's syntax borrows heavily from C and other C-like languages such as Java and Perl. Developers familiar with such languages should have an easy time picking up the somewhat looser syntax of ECMAScript.

Case-Sensitivity

The first concept to understand is that everything is case-sensitive; variables, function names, and operators are all case-sensitive, meaning that a variable named test is different from a variable named Test. Similarly, typeof can't be the name of a function because it's a keyword (described in the next section); however, typeof is a perfectly valid function name.

Identifiers

An identifier is the name of a variable, function, property, or function argument. Identifiers may be one or more characters in the following format:

• The first character must be a letter, an underscore (_), or a dollar sign ($).
• All other characters may be letters, underscores, dollar signs, or numbers.

Letters in an identifier may include extended ASCII or Unicode...

KEYWORDS AND RESERVED WORDS

ECMA-262 describes a set of reserved keywords that have specific uses, such as indicating the beginning or end of control statements or performing specific operations. By rule, keywords are reserved and cannot be used as identifiers or property names. The complete list of keywords for ECMA-262, sixth edition is as follows:

break     do        in         typeof
case      else      instanceof var
catch     export    new        void
class     extends   return     while
const     finally   super      with
continue  for       switch     yield
debugger  function  this
default   if        throw
delete    import    try

The specification also describes a set of future reserved words that cannot be used as identifiers or property names. Though reserved words don't have any specific usage in the language, they are reserved for future use as keywords.

The following is the complete list of future reserved words defined in ECMA-262, sixth edition:

Always reserved...

VARIABLES

ECMAScript variables are loosely typed, meaning that a variable can hold any type of data. Every variable is simply a named placeholder for a value. There are three keywords that can be used to declare a variable: var, which is available in all ECMAScript versions, and const and let, which were introduced in ECMAScript 6.

The 'var' Keyword

To define a variable, use the var operator (note that var is a keyword) followed by the variable name (an identifier, as described earlier), like this:

var message;

This code defines a variable named message that can be used to hold any value. (Without initialization, it holds the special value undefined, which is discussed in the next section.) ECMAScript implements variable initialization, so it's possible to define the variable and set its value at the same time, as in this example:

var message = "hi";

Here, message is defined to hold a string value of "hi". Doing this initialization doesn't...

DATA TYPES

There are six simple data types (also called primitive types) in ECMAScript: Undefined, Null, Boolean, Number, String, and Symbol. Symbol was newly introduced in ECMAScript 6. There is also one complex data type called Object, which is an unordered list of name–value pairs. Because there is no way to define your own data types in ECMAScript, all values can be represented as one of these seven. Having only seven data types may seem like too few to fully represent data; however, ECMAScript's data types have dynamic aspects that make each single data type behave like several.

The typeof Operator

Because ECMAScript is loosely typed, there needs to be a way to determine the data type of a given variable. The typeof operator provides that information. Using the typeof operator on a value returns one of the following strings:

• "undefined" if the value is undefined
• "boolean" if the value is a Boolean
• "string" if the value is a string
• "...

OPERATORS

ECMA-262 describes a set of operators that can be used to manipulate data values. The operators range from mathematical operations (such as addition and subtraction) and bitwise operators to relational operators and equality operators. Operators are unique in ECMAScript in that they can be used on a wide range of values, including strings, numbers, Booleans, and even objects. When used on objects, operators typically call the valueOf() and/or toString() method to retrieve a value they can work with.

Unary Operators

Operators that work on only one value are called unary operators. They are the simplest operators in ECMAScript.

Increment/Decrement

The increment and decrement operators are taken directly from C and come in two versions: prefix and postfix. The prefix versions of the operators are placed before the variable they work on; the postfix ones are placed after the variable. To use a prefix increment, which adds 1 to a numeric value, you place two plus signs...

STATEMENTS

ECMA-262 describes several statements (also called flow-control statements). Essentially, statements define most of the syntax of ECMAScript and typically use one or more keywords to accomplish a given task. Statements can be simple, such as telling a function to exit, or complicated, such as specifying a number of commands to be executed repeatedly.

The if Statement

One of the most frequently used statements in most programming languages is the if statement. The if statement has the following syntax:

if (condition) statement1 else statement2

The condition can be any expression; it doesn't even have to evaluate to an actual Boolean value. ECMAScript automatically converts the result of the expression into a Boolean by calling the Boolean() casting function on it. If the condition evaluates to true, statement1 is executed; if the condition evaluates to false, statement2 is executed. Each of the statements can be either a single line or a code block (a group of code...

FUNCTIONS

Functions are the core of any language because they allow the encapsulation of statements that can be run anywhere and at any time. Functions in ECMAScript are declared using the function keyword, followed by a set of arguments and then the body of the function.

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NOTE   In-depth function coverage can be found in the Functions chapter.

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The basic function syntax is as follows:

function functionName(arg0, arg1,…,argN) {
 statements
}

Here's an example:

function sayHi(name, message) {
 console.log("Hello " + name + ", " + message);
}

This function can then be called by using the function name, followed by the function arguments enclosed in parentheses (and separated by commas, if there are multiple arguments). The code to call the sayHi() function looks like this:

sayHi("Nicholas", "how are you today?");

The output of this function call is, "Hello Nicholas, how are you today?" The named arguments...

SUMMARY

The core language features of JavaScript are defined in ECMA-262 as a pseudolanguage named ECMAScript. ECMAScript contains all of the basic syntax, operators, data types, and objects necessary to complete basic computing tasks, though it provides no way to get input or to produce output. Understanding ECMAScript and its intricacies is vital to a complete understanding of JavaScript as implemented in web browsers. The following are some of the basic elements of ECMAScript:

• The basic data types in ECMAScript are Undefined, Null, Boolean, Number, String, and Symbol.
• Unlike other languages, there's no separate data type for integers versus floating-point values; the Number type represents all numbers.
• There is also a complex data type, Object, that is the base type for all objects in the language.
• A strict mode places restrictions on certain error-prone parts of the language.
• ECMAScript provides a lot of the basic operators available in C and other C-like languages, including...