JavaScript Functions

A function is a block of code with a name, that you can run many times. Functions make code shorter and easier to reuse.

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Function declaration

function greet(name) {
  return "Hello, " + name + "!";
}

console.log(greet("Robin"));
console.log(greet("Maria"));

• name is a parameter, the value you pass when you call is an argument.
• return sends a value back and stops the function. Without it the function returns undefined.

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Default parameters

function greet(name = "Guest") {
  return "Hello, " + name;
}

console.log(greet());
console.log(greet("Asha"));

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Function expression

A function can be stored in a variable.

const add = function (a, b) {
  return a + b;
};
console.log(add(2, 3));

Unlike a function declaration, a function expression is not hoisted, so you must define it before you call it.

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Rest parameters

function total(...numbers) {
  let sum = 0;
  for (const n of numbers) sum += n;
  return sum;
}
console.log(total(1, 2, 3, 4));

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Functions as values (callbacks)

In JavaScript a function is a value. You can pass it to another function. This is called a callback.

function repeat(times, action) {
  for (let i = 1; i <= times; i++) {
    action(i);
  }
}

repeat(3, function (n) {
  console.log("Round " + n);
});

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Summary

• Declare with function name() { } and call with name().
• Use return to give a value back.
• Parameters can have default values or collect all arguments with ....
• Next: Arrow Functions.

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