Speaking JavaScript

Chapter 7. JavaScript’s Syntax

JavaScript’s syntax is fairly straightforward. This chapter describes things to watch out for.

An Overview of the Syntax

This section gives you a quick impression of what JavaScript’s syntax looks like.

The following are five fundamental kinds of values:

• Booleans:

  true
  false

• Numbers:

  1023
  7.851

• Strings:

  'hello'
  "hello"

• Plain objects:

  {
      firstName: 'Jane',
      lastName: 'Doe'
  }

• Arrays:

  [ 'apple', 'banana', 'cherry' ]

Here are a few examples of basic syntax:

// Two slashes start single-linecomments

var x;  // declaring a variable

x = 3 + y;  // assigning a value to the variable `x`

foo(x, y);  // calling function `foo` with parameters `x` and `y`
obj.bar(3);  // calling method `bar` of object `obj`

// A conditional statement
if (x === 0) {  // Is `x` equal to zero?
    x = 123;
}

// Defining function `baz` with parameters `a` and `b`
function baz(a, b) {
    return a + b;
}

Note the two different uses of the equals sign:

• A single equals sign (=) is used to assign a value to a variable.
• A triple equals sign (===) is used to compare two values (see Equality Operators).

Comments

There are two kinds of comments:

• Single-line comments via // extend to the rest of the line. Here’s an example:

  var a = 0; // init

• Multiline comments via /* */ can extend over arbitrary ranges of text. They cannot be nested. Here are two examples:

  /* temporarily disabled
  processNext(queue);
  */
  
  function (a /* int */, b /* str */) {
  }

Expressions Versus Statements

This section looks at an important syntactic ...