Javascript check if argument is function

7 Powerful Methods to Check if an Argument is Passed to a JavaScript Function

Learn 7 different ways to validate function parameters and arguments in JavaScript functions. Check if an argument is passed using methods like typeof operator, if statement, arguments.length data property, and more.

• Check If Argument is Undefined
• Iterate Through Arguments and Check if Any Values Passed are Falsy
• Use the typeof Operator to Check if the Parameter is Not Equal to the String “undefined”
• Set up an If Statement and Use the Not Operator (!) to Test if the Variable Exists
• Use the arguments.length Data Property to Count How Many Arguments the Function was Called With
• Use the Value of Arguments to Access the Arguments Passed to a Function
• Use Default Parameters to Pass Default Values Instead
• Other simple code examples for checking if an argument is passed in JavaScript
• Conclusion
• How to check if an argument is passed in js?
• How to check type of argument in JavaScript?
• How do you check if a function is executed in JavaScript?
• How to check if argument is empty in JavaScript?

As a JavaScript developer, you might need to check if an argument is passed to a function. In this post, we will explore 7 different ways to check if an argument is passed to a JavaScript function. Whether you’re a beginner or an experienced JavaScript developer, this post will provide you with the knowledge to validate function parameters and arguments .

Check If Argument is Undefined

One of the most common ways to check if an argument is passed to a JavaScript function is to check if the argument is undefined. This can be done using the typeof operator. If the argument is undefined, it means that it was not passed to the function.

function myFunction(param1, param2)  if (typeof param2 === 'undefined')  console.log('param2 is not defined'); > >myFunction('value1'); 

In the example above, we check if param2 is undefined. If it is undefined, we log a message to the console.

Another way to handle this is to provide default values for arguments that are not passed to the function. This can be done using the || operator.

function myFunction(param1, param2)  param2 = param2 || 'default value'; console.log(param2); >myFunction('value1'); 

In the example above, we set a default value for param2 if it is not passed to the function.

Iterate Through Arguments and Check if Any Values Passed are Falsy

Another way to check if an argument is passed to a JavaScript function is to iterate through the arguments and check if any values passed are falsy. This method can be useful if you need to check if multiple arguments are passed to a function.

function myFunction()  for (var i = 0; i  arguments.length; i++)  if (!arguments[i])  console.log('Argument ' + (i + 1) + ' is not defined'); > > >myFunction('value1'); 

In the example above, we use a for loop to iterate through the arguments and check if any values passed are falsy. If any values passed are falsy, it means that the argument was not passed to the function.

Use the typeof Operator to Check if the Parameter is Not Equal to the String “undefined”

To check if a parameter is provided to a function , use the typeof operator to check if the parameter is not equal to the string “undefined”. This method can be useful if you need to validate function parameters.

function myFunction(param1, param2)  if (typeof param2 !== 'undefined')  console.log(param2); > >myFunction('value1'); 

In the example above, we check if param2 is not equal to undefined. If it is not equal to undefined, we log the value of param2 .

Set up an If Statement and Use the Not Operator (!) to Test if the Variable Exists

To check that an argument exists, set up an if statement and use the not operator (!) to test if the variable exists. This method can be useful if you need to check if a specific argument is passed to a function.

function myFunction(param1, param2)  if (!param2)  console.log('param2 is not defined'); > >myFunction('value1'); 

In the example above, we set up an if statement to check if param2 exists. If param2 does not exist, we log a message to the console.

Use the arguments.length Data Property to Count How Many Arguments the Function was Called With

The arguments.length data property contains the number of arguments passed to the function. You can use arguments.length to count how many arguments the function was called with. This method can be useful if you need to check how many arguments are passed to a function.

function myFunction()  if (arguments.length > 0)  console.log('Function called with ' + arguments.length + ' arguments'); > >myFunction('value1'); 

In the example above, we use the arguments.length data property to count how many arguments the function was called with. If arguments.length is greater than 0, it means that at least one argument was passed to the function.

Use the Value of Arguments to Access the Arguments Passed to a Function

The value of arguments is an array-like object corresponding to the arguments passed to a function. You can use the value of arguments to access the arguments passed to a function. This method can be useful if you need to access the arguments passed to a function.

function myFunction()  console.log(arguments[0]); >myFunction('value1'); 

In the example above, we use the value of arguments to access the first argument passed to the function.

You can also use array methods to manipulate the arguments passed to a function.

function myFunction()  var argsArray = Array.prototype.slice.call(arguments); console.log(argsArray.join(', ')); >myFunction('value1', 'value2'); 

In the example above, we use the value of arguments to create an array of arguments. We then use the join() method to join the arguments together into a string.

Remember that the arguments object is not a true array and does not support all array methods.

Use Default Parameters to Pass Default Values Instead

JavaScript functions do not check the number of arguments received. Default parameters can be used to pass the default value instead. This method can be useful if you need to set default values for function parameters.

function myFunction(param1, param2 = 'default value')  console.log(param2); >myFunction('value1'); 

In the example above, we use default parameters to set a default value for param2 if it is not passed to the function. Remember that the default values will be used if the function is called without the corresponding parameter.

Other simple code examples for checking if an argument is passed in JavaScript

In Javascript , for example, javascript check if argument is passed code sample

function func(arg1, arg2) < if (typeof arg2 === "undefined") < arg2 = "defaultValue"; >//Rest of function >

Conclusion

In this post, we have explored 7 different ways to check if an argument is passed to a JavaScript function. These methods can be useful if you need to validate function parameters and arguments. Remember to test your code with different browsers to ensure cross-browser compatibility and to handle errors gracefully when validating arguments passed to a function.

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Check if a variable is of function type or not

A JavaScript function is a block of code designed to perform a particular task. It is executed when it is invoked(when something calls it). A function can be either a named or an anonymous one. This article talks about how to go about checking whether a variable is of ‘Function’ type or not. Before we understand the different methods of implementing this and also why anyone would want to assign a function to a variable let’s look at how named and anonymous functions are declared.

Table of Contents

Function declaration types

Named Function declaration

This function has a named identifier associated with it which can be used to invoke the function

function functionName(parameter1, paramter2) < //code>

Anonymous Function declaration

It is a function that is declared without any named identifier to refer to it.

Advantage of assigning a function to a variable

Assigning a function to a variable allows us to pass this variable as a parameter to another function. This is particularly useful in scenarios that require runtime flexibility. You would mainly use such functions to run a load of code in response to an event firing For example, a button being clicked using an event handler.

myButton.onclick = function() < //response actions >

Code

Using instanceof operator

The instanceof operator is used to check the type of objects at run time. This operator returns a Boolean value(true or false). In the example below, an IF statement is used to check if the type of parameter passed to checkFunction() is of Function type or not.

//javascript check if function-Using instanceof operator < script >// Declare a variable and initialize it // Declare a variable and initialize it with an anonymous function var exampleVar = function() < /* A set of statements */ >; // to check a variable is of function type or not function checkFunction(x) < if (x instanceof Function) < document.write("Variable is of function type"); >else < document.write("Variable is not of function type"); >> // Function call checkFunction(exampleVar); < /script>

Using Strict Equality comparison (===) along with typeof operator

In JavaScript, strict equality comparison (===) Operator is used to check whether two entities are of not only equal values but also of equal type. The typeof operator returns a string which indicates the type of the unevaluated operand. Both of these operators provide a Boolean result. This result can be compared using the IF statement to check if the object type is «Function’.

//javascript check if function-Using Strict Equality comparison (===) along with typeof operator  

Using object.prototype.toString

This method uses object.prototype.toString. Every object has a toString() method, which returns ‘[object type]’ where ‘type’ is the object type. An IF statement can be used to compare if the returned value is of the type ‘Function’.

//javascript check if function-Using object.prototype.toString  

Caveats

In Chrome typeof(obj) === ‘function’ appears to be the fastest; however, in Firefox obj instanceof Function is performs relatively better.

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How to check if a JavaScript variable is a function or not.

This is a simple guide on how to check if a JavaScript variable is a function or not. As you probably already know, JavaScript allows you to create and assign functions to variables. This makes it possible to pass functions as parameters to other functions.

Let’s jump right in and take a look at the following code snippet, which includes a custom function that checks to see whether a given variable is a function or not:

//Create a JS function and assign it to a variable //called «func» var func = function()< //Print to the browser console. console.log('Hello! I am a function!'); >; //Create a simple JS string for example purposes. var str = ‘I am not a function!’; //Our custom function, which checks to see if a variable //is a function or not. function isFunction(variableToCheck) < //If our variable is an instance of "Function" if (variableToCheck instanceof Function) < return true; >return false; > //Check to see if our JavaScript variable «func» is a function. if(isFunction(func)) < //If it is, print to console and call the function. console.log('func is a function!'); func(); >else < console.log('func is not a function!'); >//Check to see if our JavaScript variable «str» is a function. if(isFunction(str)) < //If it is, print to console and call the function. console.log('str is a function!'); str(); >else

1. We create two different variables – a function type variable and a string type variable.
2. We create a custom function called isFunction, which checks to see if a JS variable is of type “function” or not. This returns TRUE or FALSE.
3. We then test both of our variables and log the results to the console.

Best of all, the custom function above will even work in old browsers such as Internet Explorer 6!

Hopefully, you found this guide to be helpful!

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