8 Using functions
This chapter covers
- Defining functions with static data types for the parameters and results
- Working with optional function parameters
- Defining function parameters with default values
- Using rest parameters to capture multiple argument values
- Overloading function types
- Using assert functions as type guards
In this chapter, I explain how TypeScript is applied to functions, showing you how TypeScript helps prevent common problems when defining functions, dealing with parameters, and producing results. Table 8.1 summarizes the chapter.
Table 8.1 Chapter summary (view table figure)
| Problem |
Solution |
Listing |
|---|---|---|
| Allow a function to be called with fewer arguments than parameters |
Define optional parameters or define parameters with default values |
7, 8 |
| Allow a function to be called with more arguments than parameters |
Use a rest parameter |
9, 10 |
| Restrict the types that can be used for parameter values and results |
Apply type annotations to parameters or function signatures |
11, 17, 18 |
| Prevent null values from being used as function arguments |
Enable the strictNullChecks compiler option |
12–14 |
| Ensure that all function code paths return a result |
Enable the noImplicitReturns compiler option |
15, 16 |
| Describe the relationship between the types of a function’s parameters and its result |
Overload the function’s types |
19, 20 |
| Describe the effect of an assert function |
Use the assert keyword |
21–23 |
For quick reference, table 8.2 lists the TypeScript compiler options used in this chapter.