- 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
This chapter is from the book
This chapter is from the book
This chapter is from the book
3.9 Testing Argument Types
In JavaScript, you do not specify the types of function arguments. Therefore, you can allow callers to supply an argument of one type or another, and handle that argument according to its actual type.
As a somewhat contrived example, the average function may accept either numbers or arrays.
const average = (x, y) => {
let sum = 0
let n = 0
if (Array.isArray(x)) {
for (const value of x) { sum += value; n++ }
} else {
sum = x; n = 1
}
if (Array.isArray(y)) {
for (const value of y) { sum += value }
} else {
sum += y; n++
}
return n === 0 ? 0 : sum / n
}
Now you can call:
result = average(1, 2) result = average([1, 2, 3], 4) result = average(1, [2, 3, 4]) result = average([1, 2], [3, 4, 5])
Table 3-1 shows how to test whether an argument x conforms to a given type.
Table 3-1 Type Tests
Type |
Test |
Notes |
|---|---|---|
String |
typeof x === 'string' || x instanceof String |
x might be constructed as new String(. . .) |
Regular expression |
x instanceof RegExp |
|
Number |
typeof x === 'number' || x instanceof Number |
x might be constructed as new Number(. . .) |
Anything that can be converted to a number |
typeof +x === 'number' |
Obtain the numeric value as +x |
Array |
Array.isArray(x) |
|
Function |
typeof x === 'function' |
|
NOTE
Some programmers write functions that turn any argument values into numbers, such as
const average = (x, y) => {
return (+x + +y) / 2
}
Then one can call
average('3', [4])
Is that degree of flexibility useful, harmless, or a harbinger of trouble? I don’t recommend it.