Understanding JavaScript Function Scope

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Did you know?

Did you know you can assign a value to a variable without declaring it?


Saving that extra bit of typing and and just assigning some value to a variable without properly declaring it can cause you a hard time by creating side effects like unexpected values.

// myUndeclaredVariable will be available here, this may not be what you want

function doSomething(){
  // This will get moved to the top of the file outside the function but will not give any error.
  myUndeclaredVariable = "Some value"

This style of coding is allowed in JavaScript but isn’t really the way you want to do things, if you used a variable with the same name in another function then you may begin to get unexpected results.

JavaScript Function Scope

JavaScript like most languages offers function scope, meaning you can declare a variable in one function and the declare another with the same name in another function. To correctly declare your variables in JavaScript, see the example below.

// A global scope variable.
var globalVariable = "Global Data"

function doSomething(){
  // A locally scoped variable.
  var localVariable = "Local Data"

The JavaScript Engine

Which ever way you declare and assign your variables the JavaScript engine will break it apart and move the declaration part to the top of the scope, so if you declare a variable with a definition on one line in a function the JavaScript engine will move the declaration to the top of the function the assignment will be left in place.

function doSomething(){
  // myPropery declaration will be hoisted to hear.

  console.log(myProperty) // output will be undefined.

  var myProperty = "Some thing"

Functions Are Hoisted Too

The way you declare your functions can matter too, you can use function declaration or function expressions when creating a function.

function doSomething() {} //Function declaration.

var myFunction = function() {} // Function expression.

The function declaration begins with the “function” keyword, the whole of this function will be hoisted, meaning we can call the function before we have declared it, so if we do something like below…


function doSomething() {
  console.log("Here is the output.");
} //Function declaration.

var myFunction = function() {}; // Function expression.

The code above will work fine because the JavaScript engine will interpret it like this.

function doSomething() {
  console.log("Here is the output.");
} //Function declaration.

var myFunction


myFunction = function() {}; // Function expression.

Notice the function declaration was moved to the top and the variable declaration for the function expression was also moved.

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