function declaration/expression differences

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  • Function declarations load before any code is executed.

  • Function expressions load only when the interpreter reaches that line of code.

So if you try to call a function expression before it’s loaded, you’ll get an error! If you call a function declaration instead, it’ll always work, because no code can be called until all declarations are loaded.

Example: Function Expression

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alert(foo()); // ERROR! foo wasn't loaded yet
var foo = function() { return 5; }

Example: Function Declaration

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alert(foo()); // Alerts 5. Declarations are loaded before any code can run.
function foo() { return 5; }

Function Declaration

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function foo() { ... }

Because of function hoisting, the function declared this way can be called both after and before the definition.

Function Expression

  1. Named Function Expression
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var foo = function bar() { ... }
  1. Anonymous Function Expression
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var foo = function() { ... }

foo() can be called only after creation.