JavaScript eval() Function

While learning JavaScript, you can come across many built-in functions that help make coding easier. One such function is the JavaScript eval() function. It is a powerful tool, but it can be risky if not used properly. In this blog, you will learn how to use eval() in JavaScript, how it works, and where you should use it in detail with examples.

Table of Contents

• What is eval() in JavaScript?
  • Syntax of eval() in JavaScript
• Parameters of eval() in JavaScript
• Return value of eval() in JavaScript
• How eval() handles non-string inputs and errors
• How eval() Works in JavaScript?
• Examples of eval() in JavaScript
• Why Avoid eval() in JavaScript
• Alternatives to eval() in JavaScript
• Best Practices for eval() in JavaScript
• Conclusion

What is eval() in JavaScript?

The eval() function in JavaScript is a built-in function that allows you to evaluate and execute a specific part of JavaScript code written as a string. In simple words, if you pass a string of code inside eval(), JavaScript will treat it like actual code and run it. Even though the eval() function can be useful at times, you should avoid using it because it can make your code slow and unsafe.

Syntax of eval() in JavaScript

The eval() function in JavaScript takes a string as input and executes it as JavaScript code. If the string is invalid, eval() throws an error.

Syntax: 

eval(string)

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Parameters of eval() in JavaScript

The eval() function in JavaScript takes only one parameter, which must be a string. This string should contain valid JavaScript code. If the string is valid, then eval() will run the string like normal JavaScript code. But if the string is not valid, JavaScript will show an error.

Code:

Output:

Explanation: In the above JavaScript code, the first line runs the code and returns 5, while the second line fails because hello is not defined in the code.

Return value of eval() in JavaScript

The eval() function in JavaScript is used to return the result of the code that is executed. If the code is an expression, it returns the value of that expression. If the code does not return anything (like a variable declaration), then the eval() returns undefined.

Example:

Output:

Explanation: Here, the return value of eval() depends on the kind of code you have passed inside it.

How eval() handles non-string inputs and errors

If something else is passed inside eval() in JavaScript other than a string, it just returns the same thing without doing anything to it.

Example:

Output:

Explanation: In the above code example, we passed an array to the eval() function. Since it wasn’t a string, eval() sent the same array back without changing it.

Example:

Output:

Explanation: In the above code example, “a * 7” is a string that is passed to the eval() function. When you run it, it takes the value of a, multiplied by 7, and then gives the result. The result is saved in the output variable and shown on the webpage.

How eval() Works in JavaScript?

The eval() function in JavaScript works by taking a string and running it just like a normal JavaScript code. At first, it checks the string to see if the code is valid. If it is, JavaScript executes the code and gives the result. If the code is not valid, the eval() function throws an error.

Example:

Output:

Explanation: In the above code, the string “a * 10” is passed to the eval() function. The eval() takes the string as actual code. After that, the value is multiplied by 10. It returns the result (50). Therefore, the eval() function in JavaScript is used to turn strings into executable JavaScript code.

Examples of eval() in JavaScript

In this section, we will discuss a few more examples of eval() in JavaScript so that you can get a complete understanding of the topic

1. Using eval() to Call a Function

Here, you will see how the JavaScript eval() function is used to call a function dynamically. Using eval(), you can run a function by passing its name as a string.

Example:

Output:

Explanation: The above JavaScript code uses the eval() function to run the mean function with 20 and 13. After that, it stores and prints the result.

2. Using eval() with JavaScript Statements

Here, we will discuss how the JavaScript eval() function can run full JavaScript statements and not just expressions. It also allows you to execute code written in a string directly.

Code:

Output:

Explanation: The above JavaScript code used the eval() function to run an if-else statement, which is written as a string. After that, it displays the message based on the value of time.

3. Passing a String Argument in eval()

Here, will see how the JavaScript eval() function can take a string as an argument and run it as code. This allows you to execute the expressions or statements directly in the strings.

Code:

Output:

Explanation: The above code uses the JavaScript eval() function to calculate the value of the string “4 + 11”. After that, the result is printed.

4. Using eval() with JSON Objects

In this example, you will see how the JavaScript eval() function can work with JSON objects. Using the eval() function, you can convert a JSON string into an actual JavaScript object that you can use in your code.

Code:

Output:

Explanation: The above code uses the JavaScript eval() function to turn a JSON-like string into an object. After that, it prints the name of the company, profile, and the salary.

Note: The eval() executes any code and can be exploited if the string comes from an untrusted source. For JSON data, always use JSON.parse() in JavaScript, which safely parses the string into a JavaScript object.

Why Avoid eval() in JavaScript

1. Security Risk: Using the JavaScript eval() function along with user input allows hackers to run harmful code on your website.

2. Slower Performance: Using the JavaScript eval() function will make your code slower. This is because it forces JavaScript to read and execute the string again and again.

3. Hard to debug: The code inside the eval() function in JavaScript is harder to read and debug. This makes it difficult to maintain your JavaScript projects.

4. Unnecessary Complexity: The tasks that are done using the JavaScript eval() function can be done more easily with other methods like functions, JSON parsing, or loops.

5. Can Cause Errors: If the string passed to the JavaScript eval() function has a mistake, it can affect your program and throw unexpected exceptions.

Alternatives to eval() in JavaScript

Here, we will discuss the best alternatives to the eval() function in JavaScript.

1. JSON.parse() for JSON Handling

Instead of using the JavaScript eval() function to convert JSON strings into objects, you can use the JSON.parse(). This method works only with valid JSON, which makes it a much safer option.

Example:

Output:

Explanation: The above code is used to convert a JSON string into a JavaScript object by using JSON.parse(). After that, it prints the value of the name property, which is “Rahul”.

2. Function Constructor

For safely executing dynamic code, you can use the Function() constructor instead of using the JavaScript eval() function. It helps to create a new function from a string, but it doesn’t access variables, which helps to reduce risk.

Example:

Output:

Explanation: The above JavaScript code is used to create a function using the Function() constructor. After that, it multiplies two numbers, and calls 5 and 10 to print 50.

3. Safer Approaches for Dynamic Code Execution

In most cases, where developers use the JavaScript eval() function, there are safer alternatives to it as well:

• You can use predefined functions instead of generating code as strings.
• You can also use JSON.parse() for working with data.
• You can also use array methods, loops, or properties of objects dynamically without using eval().

These methods allow you to handle the dynamic tasks in a safe way without the security and performance risks of eval().

Best Practices for eval() in JavaScript

Given below are the 5 best practices for eval() in JavaScript:

1. Avoid using eval() whenever possible: You should try to solve the problems with safer methods like functions or JSON.parse().

2. Never use eval() with user input: You should never use eval() with user input, as it may allow attackers to run harmful code in your program.

3. Use eval() only in controlled cases: If it is compulsory to use the eval() function in JavaScript, then you need to make sure that the code inside is safe and trusted.

4. Prefer alternatives for performance: Methods like loops, object access, or constructors run faster and safer than eval().

5. Keep code simple and clear: Using eval() makes code harder to read, so it is better to stick to straightforward solutions.

Conclusion

The JavaScript eval() function may look powerful because it can turn strings into executable code, but you should now understand the use of eval in JavaScript and why it comes with risks. While the JavaScript eval() function can run expressions, statements, or even call functions, its downsides are security problems, slower performance, and harder debugging. Instead of depending on the JavaScript eval function, it is better to use safer options like JSON.parse() for handling JSON, or the Function() constructor for dynamic code. By knowing when and how to use or avoid eval(), you can write cleaner, faster, and more secure JavaScript code.

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JavaScript eval() Function – FAQs