Functions and Functional Programming in JavaScript
- 3.1 Declaring Functions
- 3.2 Higher-Order Functions
- 3.3 Function Literals
- 3.4 Arrow Functions
- 3.5 Functional Array Processing
- 3.6 Closures
- 3.7 Hard Objects
- 3.8 Strict Mode
- 3.9 Testing Argument Types
- 3.10 Supplying More or Fewer Arguments
- 3.11 Default Arguments
- 3.12 Rest Parameters and the Spread Operator
- 3.13 Simulating Named Arguments with Destructuring
- 3.14 Hoisting
- 3.15 Throwing Exceptions
- 3.16 Catching Exceptions
- 3.17 The finally Clause
- Exercises
JavaScript has an object model that is very different from class-based programming languages. Learn about functions and functional programming, which is very important in JavaScript.
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This chapter is from the book
This chapter is from the book
This chapter is from the book
Topics in This Chapter
3.1 Declaring Functions
3.2 Higher-Order Functions
3.3 Function Literals
3.4 Arrow Functions
3.5 Functional Array Processing
3.6 Closures
3.7 Hard Objects
3.8 Strict Mode
3.9 Testing Argument Types
3.10 Supplying More or Fewer Arguments
3.11 Default Arguments
3.12 Rest Parameters and the Spread Operator
3.13 Simulating Named Arguments with Destructuring
3.14 Hoisting
3.15 Throwing Exceptions
3.16 Catching Exceptions
3.17 The finally Clause
Exercises
In this chapter, you will learn how to write functions in JavaScript. JavaScript is a “functional” programming language. Functions are “first-class” values, just like numbers or strings. Functions can consume and produce other functions. Mastering a functional programming style is essential for working with modern JavaScript.
This chapter also covers the JavaScript parameter passing and scope rules, as well as the details of throwing and catching exceptions.
3.1 Declaring Functions
In JavaScript, you declare a function by providing
The name of the function
The names of the parameters
The body of the function, which computes and returns the function result
You do not specify the types of the function parameters or result. Here is an example:
function average(x, y) {
return (x + y) / 2
}
The return statement yields the value that the function returns.
To call this function, simply pass the desired arguments:
let result = average(6, 7) // result is set to 6.5
What if you pass something other than a number? Whatever happens, happens. For example:
result = average('6', '7') // result is set to 33.5
When you pass strings, the + in the function body concatenates them. The resulting string '67' is converted to a number before the division by 2.
That looks rather casual to a Java, C#, or C++ programmer who is used to compile-time type checking. Indeed, if you mess up argument types, you only find out when something strange happens at runtime. On the flip side, you can write functions that work with arguments of multiple types, which can be convenient.
The return statement returns immediately, abandoning the remainder of the function. Consider this example—an indexOf function that computes the index of a value in an array:
function indexOf(arr, value) {
for (let i in arr) {
if (arr[i] === value) return i
}
return -1
}
As soon as a match is found, the index is returned and the function terminates.
A function may choose not to specify a return value. If the function body exits without a return statement, or a return keyword isn’t followed by an expression, the function returns the undefined value. This usually happens when a function is solely called for a side effect.
TIP
If a function sometimes returns a result, and sometimes you don’t want to return anything, be explicit:
return undefined
NOTE
As mentioned in Chapter 2, a return statement must always have at least one token before the end of the line, to avoid automatic semicolon insertion. For example, if a function returns an object, put at least the opening brace on the same line:
return {
average: (x + y) / 2,
max: Math.max(x, y),
. . .
}