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Function Parameters

A JavaScript function does not check the parameter values (arguments).

Function Parameters and Arguments

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 functions do not specify data types for parameters.

JavaScript functions do not perform checks on the passed arguments.

JavaScript functions do not check the number of arguments retrieved.

Default Parameters

If a function is called without any arguments (less than declared), the missing values are set to undefined.

At times, it is acceptable, but it is better to assign a default value to the parameter:

Example

<!DOCTYPE html>

<html>

<body>

<p>Setting a default value to a function parameter.</p>

<p id=”demo”></p>

<script>

function myFunction(x, y) {

  if (y === undefined) {

    y = 2;

  } 

  return x * y;

}

document.getElementById(“demo”).innerHTML = myFunction(4);

</script>

</body>

</html>

Output

Setting a default value to a function parameter.

The Arguments Object

JavaScript functions consist of a built-in object called the arguments to object.

The argument object has an array of the arguments used when the function was called (invoked).

You can simply use a function to find (for instance) the highest value in a list of numbers.

Example

<!DOCTYPE html>

<html>

<body>

<h2>JavaScript Functions</h2>

<p>Finding the largest number.</p>

<p id=”demo”></p>

<script>

function findMax() {

  let max = -Infinity;

  for(let i = 0; i < arguments.length; i++) {

    if (arguments[i] > max) {

      max = arguments[i];

    }

  }

  return max;

}

document.getElementById(“demo”).innerHTML = findMax(4, 5, 6);

</script>

</body>

</html>

Output

JavaScript Functions

Finding the largest number.

6

Arguments are Passed by Value

  • The parameters, in a function call, are the function’s arguments.
  • JavaScript arguments are passed through value.
  • If a function changes an argument’s value, it does not alter the parameter’s original value.
  • Any changes to arguments are not reflected outside the function.

Objects are Passed by Reference

  • In JavaScript, object references are values.
  • Because of this, objects will behave as if they are passed by reference:
  • If a function changes an object’s property, it alters the original value.
  • Any changes to object properties are reflected outside the function.

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