function declaration vs function expression

In JavaScript, there are some different ways to create a function. This post will talk about the difference between the following ways:
// Function expression
var fn = function() {
    // The implementation ...
}

// Function declaration
function fn() {
    // The implementation ...
}

Difference

A function created by using an expression is assigned to a variable at run time. We cannot invoke the function before the declaration.
// TypeError: sum is not a function
sum(4, 2); 

var sum = function(a, b) {
    return a + b;
}
Function declaration on the other hand is assigned to an identifier at parse time. It is available above its declaration.
sum(4, 2);  // 6 

function sum(a, b) {
    return a + b;
}

Good to know

  1. In the strict mode introduced in ES5, function declaration is scoped to its enclosing block. We cannot invoke the function from outside of the block.
    'use strict';    
    {
        function sum(a, b) {
            return a + b;
        }
    }
    
    // ReferenceError: sum is not defined
    sum();
  2. We can mix the two variants by using a named function expression. With this variant, the function name is taken from the declaration form.
    var sum = function sumOf(a, b) {
        return a + b;
    }
    
    sum.name;   // "sumOf"
  3. Usually, we can invoke a function by using the form of Immediately Invoked Function Expression (IIFE).
    (function(a, b) {
        return a + b;
    })(4, 2);
    
    // 6
    Do you know that we get the same result if we omit the parentheses and put + at the beginning as follow:
    +function(a, b) {
        return a + b;
    }(4, 2)
    
    // 6
    It works because putting + at the beginning of function declaration will turn it to an expression, and passing the parameters with () at the end will invoke the expression.
    It is rare to see that code in development, but it is used in the minifications to save the file size.
    In addition to +, you can use other operators such as -, !, ~ and void in the similar way to invoke a function:
    -function() { ... }();
    !function() { ... }();
    ~function() { ... }();
    void function() { ... }();
    Note that the return value could be different from the original function, for example:
    !function() { return false; }();     // true
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