You can use a function declaration or a function expression.
Earlier in this tutorial, you learned that functions are declared with the following syntax:
Declared functions are not executed immediately. They are "saved for later use", and will be executed later, when they are invoked (called upon).
Since a function declaration is not an executable statement, it is not common to end it with a semicolon.
A function expression can be stored in a variable:
After a function expression has been stored in a variable, the variable can be used as a function:
The function above is actually an anonymous function (a function without a name).
Functions stored in variables do not need function names. They are always invoked (called) using the variable name.
The function above ends with a semicolon because it is a part of an executable statement.
You actually don't have to use the function constructor. The example above is the same as writing:
Most of the time, you can avoid using the
Hoisting applies to variable declarations and to function declarations.
Functions defined using an expression are not hoisted.
Function expressions can be made "self-invoking".
A self-invoking expression is invoked (started) automatically, without being called.
Function expressions will execute automatically if the expression is followed by ().
You cannot self-invoke a function declaration.
You have to add parentheses around the function to indicate that it is a function expression:
The function above is actually an anonymous self-invoking function (function without name).
arguments.length property returns the number of arguments received when the function was invoked:
toString() method returns the function as a string:
A function defined as the property of an object, is called a method to the object.
A function designed to create new objects, is called an object constructor.
Arrow functions allows a short syntax for writing function expressions.
You don't need the
function keyword, the
return keyword, and the curly brackets.
Arrow functions do not have their own
this. They are not well suited for defining object methods.
Arrow functions are not hoisted. They must be defined before they are used.
const is safer than using
var, because a function expression is always constant value.
You can only omit the
return keyword and the curly brackets if the function is a single statement. Because of this, it might be a good habit to always keep them:
Arrow functions are not supported in IE11 or earlier.