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JavaScript Function Parameters


A JavaScript function does not perform any checking on
parameter values (arguments).

Function Parameters and Arguments
Earlier in this tutorial, you learned that functions can have
parameters:


function functionName(parameter1, parameter2, parameter3) {
  // code to be executed
}


Function parameters are the names listed in
the function definition.
Function arguments are the real values
passed to (and received by) the function.

Parameter Rules
JavaScript function definitions do not specify data types for
parameters.
JavaScript functions do not perform type checking on the passed
arguments.
JavaScript functions do not check the number of arguments received.

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

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

}

Try it Yourself »


Default Parameter Values
ES6 allows function parameters to have default values.

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

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

Try it Yourself »



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

Example

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);

Try it Yourself »








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

x = findMax(1, 123, 500, 115, 44, 88);function findMax() {
  let max = -Infinity;
  for (let i = 0; i < arguments.length; i++) {
    if (arguments > max) {
      max = arguments;
    }
  }
  return max;
}

Try it Yourself »

Or create a function to sum all input values:

Example

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

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

Try it Yourself »


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


Arguments are Passed by Value
The parameters, in a function call, are the function's arguments.
JavaScript arguments are passed by value: The function only
gets to know the values, not the argument's locations.
If a function changes an argument's value, it does not change the parameter's
original value.
Changes to arguments are not visible (reflected) outside the function.

Objects are Passed by Reference
In JavaScript, object references are values.
Because of this, objects will behave like they are passed by reference:
If a function changes an object property, it changes the original value.
Changes to object properties are visible (reflected) outside the function.












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Reference: https://www.w3schools.com/js/js_function_parameters.asp