Javascript · Functions (Advanced)
Default Parameters
The full text of this chapter, to read or revisit. To practise, do the exercises.
▶ Do the exercisesExercise 1 — A Value When Nothing Is Passed
Remember: a parameter without an argument is `undefined`. That gives awkward messages:
function greet(name) {
console.log("Welcome, " + name + "!");
}
greet("Sam");
greet();
A default parameter fixes this: write `= value` after the parameter name. That value is used whenever no argument is passed.
function greet(name = "trainer") {
console.log("Welcome, " + name + "!");
}
greet("Sam");
greet();
Exercise 2 — Defaults for Numbers
Defaults are handy for optional settings. Here a potion heals 20 HP unless you say otherwise:
function heal(hp, amount = 20) {
return hp + amount;
}
console.log(heal(50));
console.log(heal(50, 35));
Defaults work in every kind of function: declarations, expressions and arrow functions.
const attack = (damage = 10) => "Deals " + damage + " damage";
console.log(attack());
console.log(attack(45));
Exercise 3 — Quiz: Default Used?
What does this code print?
function level(n = 1) {
return "Level " + n;
}
console.log(level());
console.log(level(7));
Options
• ```Level 1``` then ```Level 7```
• ```Level undefined``` then ```Level 7```
• ```Level 1``` twice
• An error
Hint : The default only steps in when nothing is passed.
Exercise 4 — Default Damage
Give `damage` a default of 15, so that the first call prints 15 and the second prints 40.
function hit(damage ____ ____) {
console.log(damage);
}
hit();
hit(40);
Options: = 15 == 40 :
Hint : A default is written like an assignment, right in the parameter list.
Exercise 5 — undefined Triggers the Default
Precisely, the default is used when the argument is `undefined`: missing, or explicitly `undefined`. Any other value, even `0`, `""` or `null`, is kept as is.
function show(value = "default") {
console.log(value);
}
show();
show(undefined);
show(0);
show("");
show(null);
The third line prints `0` and the fourth prints an empty line: those are real values, not "nothing".
Exercise 6 — Quiz: Zero Is a Value
What does this code print?
function bonus(points = 50) {
return points;
}
console.log(bonus(0));
Options
• ```0```
• ```50```
• ```undefined```
• An error
Hint : Is 0 the same thing as "nothing was passed"?
Exercise 7 — Order Matters
Arguments are matched by position, so put parameters with defaults last. Otherwise you can't skip them:
function introduce(name, type = "unknown", level = 1) {
console.log(name + " | " + type + " | level " + level);
}
introduce("Zap");
introduce("Zap", "electric");
introduce("Zap", "electric", 5);
A default can even be computed from a previous parameter:
function stats(level, hp = level * 10) {
console.log("Level " + level + ", " + hp + " HP");
}
stats(4);
stats(4, 99);
Exercise 8 — Optional Level
Complete the parameters so the level defaults to 1 and the two calls print `Zap Lv.1` and `Metachu Lv.12`.
const label = (____, level ____ 1) => name + " Lv." + level;
console.log(label("Zap"));
console.log(label("Metachu", 12));
Options: name = level == "name"
Hint : The required parameter comes first, and the optional one carries its default.
Exercise 9 — Default Cheat Sheet
`function f(a, b = 5)` is called in different ways. Match each call with the value of `b`.
Items to match
• f(1)
• f(1, 8)
• f(1, undefined)
• f(1, 0)
Exercise 10 — Flexible Potion
Write a `usePotion` function that heals a Codémon, with sensible defaults so most calls stay short.
Step 1: `usePotion(name, hp, amount = 20, max = 100)` returns the new HP: the current HP plus the amount, but never above the max.
Step 2: it also prints `"name: X -> Y HP"` where X is the HP before and Y the HP after.
Step 3: make the three calls shown below, using the defaults whenever possible.
Calls: Zap at 50 HP with a standard potion; Pyrochu at 90 HP with a standard potion; Metachu at 100 HP with a 50 HP potion and a max of 140.
Zap: 50 -> 70 HP
Pyrochu: 90 -> 100 HP
Metachu: 100 -> 140 HP
Starter code
// Write your function here
// Make the three calls here