11.2 Function Parameters

Default Parameters

• If a function is called with missing arguments (less than declared), the missing values are set to undefined.
• Sometimes this is acceptable, but sometimes it is better to assign a default value to the parameter:

Example 1: JS Default Parameters

function myFunction(x, y) {
  if (y === undefined) {
    y = 2;
  }
}

Default Parameter Values

ES6 allows function parameters to have default values.

Example 2: JS Functions - Default Parameter Values

If y is not passed or is undefined, then y = 10.

function myFunction(x, y = 10) {
  return x + y;
}
myFunction(5);

Function Rest Parameter

The rest parameter (...) allows a function to treat an indefinite number of arguments as an array:

Example 3: JS functions - The Rest Parameter

function sum(...args) {
  let sum = 0;
  for (let arg of args) sum += arg;
  return sum;
}

let x = sum(4, 9, 16, 25, 29, 100, 66, 77);

The Arguments Object

• JavaScript functions have a built-in object called the arguments object.
• The argument object contains an array of the arguments used when the function was called (invoked).
• This way you can simply use a function to find (for instance) the highest value in a list of numbers:

Example 4: JS Functions - Finding the largest number

x = findMax(1, 123, 500, 115, 44, 88);

function findMax() {
  let max = -Infinity;
  for (let i = 0; i < arguments.length; i++) {
    if (arguments[i] > max) {
      max = arguments[i];
    }
  }
  return max;
}

Or create a function to sum all input values:

Example 5: JS Functions - Sum of all arguments

x = sumAll(1, 123, 500, 115, 44, 88);

function sumAll() {
  let sum = 0;
  for (let i = 0; i < arguments.length; i++) {
    sum += arguments[i];
  }
  return sum;
}

If a function is called with too many arguments (more than declared), these arguments can be reached using the arguments object.