JavaScript Function Arguments
Parameters vs. Arguments
In JavaScript, function parameters and arguments are distinct concepts:
Parameters are the names listed in the function definition.
Arguments are the real values passed to, and received by the function.
In the example above:
- a and b are parameters
- 4 and 5 are arguments
The argument 4 is assigned to the parameter a.
The argument 5 is assigned to the parameter b.
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[i] > max) {
max = arguments[i];
}
}
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[i];
}
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.
The Order of Arguments Matters
Arguments are assigned to parameters in the order they appear.
Example
function subtract(a, b) {
return a - b;
}
let x1 = subtract(10, 5);
let x2 = subtract(5, 10);
Try it Yourself »
The two calls above return different results because the order is different.
Arguments Can Be Variables
Arguments do not have to be values. They can also be variables.
Example
let x = 5;
let y = 6;
function multiply(a, b) {
return a * b;
}
multiply(x, y);
Argument Rules
JavaScript function definitions do not specify data types for arguments.
JavaScript functions do not perform type checking on the passed arguments.
JavaScript functions do not check the number of arguments received.
Incorrect Arguments
Incorrect arguments can return incorrect answers:
Example
function toCelsius(fahrenheit) {
return (5/9) * (fahrenheit-32);
}
let value = toCelsius("John");
Try it Yourself »
Missing Arguments
If a function is called with fewer arguments than parameters, the missing values become undefined.
Example
function multiply(a, b) {
return a * b;
}
multiply(4);
In the example above, b is undefined, so the result is NaN.
Note
A JavaScript function does not perform any checking on parameter values (arguments).
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:
Default Parameter Values
ECMAScript 2015 allows function parameters to have default values.
You can now set a default value for a parameter.
The default value is used if no argument is provided.
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 »
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.
Common Mistakes
Confusing Parameters and Arguments
Parameters are names.
Arguments are values.Forgetting the Order
Arguments are assigned by position.Missing Arguments
Use default values to avoid undefined.
Quiz
What happens when a function is called with fewer arguments than parameters?
Next Chapter
Next: Function Expressions
