Difference between `var functionName = function() {}` and `functionName() {}` in JavaScript
David Y.
—What is the difference between var functionName = function() {}
and functionName() {}
in JavaScript?
We call var functionName = function() {}
a function expression and functionName() {}
a function declaration.
Function expressions are only evaluated when the JavaScript interpreter reaches their line of code, whereas function declarations are evaluated at the start of execution. We can see the difference in this code snippet:
functionExpressed(); // will throw a TypeError, as functionExpressed is not yet defined var functionExpressed = function() { console.log('Executing functionExpressed...'); }; functionExpressed(); // will print "Executing functionExpressed..." as it is now defined functionDeclared(); // will print "Executing functionDeclared..." even though we only define the function below this call function functionDeclared() { console.log('Executing functionDeclared...'); }
In most simple cases, we will want to use function declarations, but it can sometimes be useful to have a function that is only defined after other code has run. Function expressions give us more control over the code flow at the expense of having to be more careful about where we place individual statements.
Tasty treats for web developers brought to you by Sentry. Get tips and tricks from Wes Bos and Scott Tolinski.
SEE EPISODESConsidered “not bad” by 4 million developers and more than 100,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.