Lesson 5
Parameters & returns
Build a practical mental model for parameters & returns and use it in working JavaScript.
By the end, you will be able to…
- Explain parameters & returns in your own words
- Use parameters & returns in a small program
- Recognize and debug a common failure mode
Build the mental model
Default parameters supply a fallback when an argument is omitted, and every meaningful function should return a value the caller can use rather than only logging internally.
Syntax and working example
Read the code from inputs to output. Predict what it prints, then run it and change one value.
Why this matters
Functions that return values compose — you can pass one function's output straight into another — while functions that only log cannot.
Real-world example
Utility libraries like date and currency formatters are built entirely from small functions that take inputs and return a usable result.
Common mistakes
- Logging inside a function instead of returning, making the result unusable elsewhere
- Forgetting a default value and getting undefined for optional arguments
- Mutating a parameter instead of returning a new value
Try it yourself
Challenge
Add a default 'decimals' parameter that controls how many digits formatPrice shows.
Knowledge check
Why should most functions return a value instead of only calling console.log?
Summary
Give parameters sensible defaults and always return a usable value instead of only logging.