Difference between `var functionName = function() {}` and `functionName() {}` in JavaScript

David Y.

The Problem

What is the difference between var functionName = function() {} and functionName() {} in JavaScript?

The Solution

We call var functionName = function() {} a function expression and functionName() {} a function declaration.

• We use a function expression to assign a function as the value of a variable.
• We use a function declaration to declare a function to be used anywhere in our scope.

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...');
}
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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.