eval in javascript

What is eval in javascript and Best 4 javascript eval examples

The Eval in JavaScript is a built-in global function, which means it can be called directly without the need for an Object. The purpose of this function is to evaluate JavaScript expressions or a set of commands stored in the “String” data type (String | String).

This function is particularly useful for evaluating string-based user inputs that represent JavaScript expressions. In this article from the thinkwriteway.com blog, we first define the Eval function and learn about its syntax. Then we will check the parameters, return value, and its possible exceptions and review several examples. Finally, we will check the support of browsers for this function. So, join us to learn about the Eval function in the JavaScript programming language.

What is Eval in JavaScript?

The Eval function in JavaScript is a type of global function that evaluates a string containing an expression, command, or sequence of JavaScript expressions. It can be used to evaluate expressions that come as string input from users. The Eval function in JavaScript takes a string as its argument. The line of text can have any correct JavaScript instructions. The Eval function then evaluates the string as JavaScript code and returns the result.

The Eval in javascript function can be used for the following purposes:

  • Evaluating expressions that come as string input from users.
  • Dynamic content production
  • Debugging JavaScript code
eval in javascript

It is also important to note that the function Eval () is also a security risk because it can be used to execute malicious code.

Eval syntax in JavaScript

The ” Syntax “ of using the Eval function in Javascript is as follows:

eval(expression);

In the code above, the term expression refers to a string that contains a single statement or a sequence of statements. For example, the expression can look like “4 + 11” Or even something like a simple message “Hello” be

Eval function parameters in Javascript

The Eval function in JavaScript takes a single parameter, which is a string containing an expression, a command, or a sequence of JavaScript expressions. An expression or command can contain variables, functions, or even JavaScript objects. For example, the code below the statement “1 + 2” evaluates:

const result = eval("1 + 2");

The code below the statement console.log (“Hello, world!”) evaluates and implements

eval("console.log('Hello, world!');");

The following code evaluates a sequence of expressions.

eval("var x = 1; x++;");

The return value of Eval in Javascript

Using a string value as an argument, the Eval function in JavaScript returns the return value corresponding to the evaluation result of the provided code. On the other hand, if no definite value appears from the evaluation process, the function Eval()  ”undefined” value ( Undefeated ) returns. For example, the code below the statement “1 + 2” evaluates and the value 3 displays in the output:

const result = eval("1 + 2");

The code below the statement console.log (“Hello, world!”) does a task, but it doesn’t give back any result.

eval("console.log('Hello, world!');");

The code below is a sequence of statements var x = 1 evaluates and the value x returns that 2 Is:

eval("var x = 1; x++;");

Eval exceptions in JavaScript language

When non-string arguments are used for the Eval function in the JavaScript programming language, it exhibits special behaviors. In such cases, the function returns essentially the same argument it received. The following example shows the concept of using non-string arguments for the Eval function in JavaScript language:

let arr = [1, 2, 3];
// Utilizing the eval() function
let output = eval(arr);
// Displaying the resulting output
console.log(output);

The outcome of the code you see above appears like this:

[1, 2, 3]

In the above scenario, an array is an argument to the function Eval() Provided. Since the supplied argument is not of the string data type, the function will obviously return the original array unchanged. Paying attention to the following example is very important to express this issue and its code fragment is as follows:

var a = 3;

var multiply_str;
var output;

// writing a string that multiplies 2 variables
multiply_str = "a * 7";

// using the eval() function
output = eval(multiply_str);

// printing the result
console.log(output);

The outcome of the code you see above appears like this:

21

In the above scenario, a string is created to represent the multiplication of two variables. That is, the value stored in the variable “a” f number 7 . Using the function Eval(), this string expression will be evaluated. As a result, the function executes the operation which is the multiplication of two numbers. The result of this operation subsequently to the variable output allocated and displayed through the console.

How does Eval work in JavaScript?

The main operation of the Eval function in the JavaScript programming language is to receive a string-based argument. If this string contains an expression, the Eval in javascript function evaluates and calculates the expression. Conversely, if the string contains an expression, the function evaluates the expression. Despite the potential utility of the Eval function in JavaScript, it is generally recommended that caution be exercised in its use and that its use be kept to a minimum. Below is the reason why you should be careful in using this JavaScript function.

The 3 main reasons for not using this function are as follows:

  • Execution speed: the execution speed of the function Eval()It is noticeably slow due to the activation of the JavaScript interpreter. This function can affect the performance of the program and lead to a decrease in the speed of the program.
  • Security vulnerabilities: when the function Eval() The argument processes a string that holds the malicious code, opening the door to possible malware attacks on the developer’s system or server. This is a very significant security risk that users should be aware of.
  • Reduce code readability: function inclusion Eval() In the user code, it can lead to a decrease in the readability of the code. By itself, this function can increase the complexity of the code, obfuscate the logical flow of the code, and make it more challenging to understand and maintain.
eval in javascript

Due to these inherent problems in using the mentioned function, it is recommended to use caution and alternative approaches instead of using the Eval function in JavaScript.

Examples of the Eval function

In this section, some examples are given that demonstrate the use of the Eval function in JavaScript, and their purpose is to provide a comprehensive understanding of the subject.

Example 1: Using the Eval function to call the function

Consider the following example.

// defining a function
function mean(a, b) {
    return (a + b) / 2;
}

// creating a variable to store the result
let ans;

// using the eval() function
ans = eval("mean(20, 13)");

// displaying the result
console.log(ans);

The outcome of the example you see above appears like this:

16.5

In the above example, a function with two arguments using a string inside the function Eval() It is encapsulated. The resulting value of Eval() in the variable and is saved and subsequently printed on the console.

Example 2: Using the Eval function with JavaScript commands

In the continuation of this article from thinkwriteway.com blog, another example of application Eval()We provide JavaScript commands.

// creating variables
let time = 7;
let statement_str;

// writing the if else conditional inside a string
statement_str = "if(time >= 6 && time <= 10) {'Time to wake up!';} else {'You are late.';}"

// displaying the output
console.log(eval(statement_str));

The outcome of the example you see above appears like this:

Time to wake up!

In the above example, some kind of conditional statement is formulated in the string. Then the string as an argument to the function Eval() will provide. Because variable “time” Has value 7 the output “Time to wake up!” is produced.

Example 3

In the third example, actions Eval() The argument to the string object is checked.

// creating a variable and using eval() function
let output = eval(new String("4 + 11"));

// displaying the output
console.log(output);

The outcome of the example you see above appears like this:

"4 + 11"

In the example above, the string object is the argument to the function Eval() Provided. As a result of sending the string object, the function Eval() Skips evaluating the content and returns the original argument unchanged So, what comes out is “4 + 11” instead of the real answer, which is 15.

eval in javascript

Example 4 – Use of Eval() with ” JSON ” object

// creating a variable
let object_str;

// creating an object string
object_str = "({'Company': 'Google', 'Profile': ['Backend Dev.', 'Software Dev.'], 'Salary': [100000, 120000]})";

// using the eval() function to convert the string to a JS object
const job_desc = eval(object_str);

// displaying the result
console.log("The salary at ", job_desc.Company, " for a ", job_desc.Profile[0], " is ", job_desc.Salary[0]);

This is output from example 4:

The salary at Google for a Backend Dev. is 100000

In the above example, the variable object_str has a string representation similar to a JSON object. Using the function Eval(), the string is converted to some kind of real javascript object, and then in the variable job_desc is saved. As a result, object properties such as «job_desc.Company»، «job_desc.Profile» etc. It can help make useful stuff.

Eval function is supported in which browsers?

The Eval function is supported in many modern web browsers, the most important of which are Chrome and Microsoft Edge. Other browsers are listed below:

  • Mozilla Firefox (version 1 and later versions)
  • Safari (version 1 and later versions)
  • Opera (version 3 and later versions)
  • Internet Explorer (version 3 and later versions)

Conclusion

The Eval function in JavaScript works to evaluate the provided string containing the expression or command. When given a string containing an expression or command, it calculates the resulting value and returns the result. If no value is specified, the output of the function will be undefined. Also, when a non-string argument is used in the function Eval() Returns the argument unchanged.

It is generally recommended that this eval function in JavaScript be used less frequently in applications due to the potential for slow execution and security vulnerabilities. Despite these drawbacks, Eval() can be useful for calling functions and interacting with objects in JavaScript in some scenarios. In the above article from the thinkwriteway technology blog, relatively favorable information about the Eval in JavaScript was provided, including its syntax, how to use it, its parameters, return value, and reasons for not using it. I hope that the above content has been satisfied by dear users.

If you seek further information about the What is eval in javascript and Best 4 javascript eval examples or have any inquiries regarding our blog, please don’t hesitate to contact us via email at info@thinkwriteway.com

What are Variables in JavaScript and the Best 25 Examples

What are Constructor Functions in Javascript and The Best 15 Examples?

Share This Article :

Leave a Comment

What to read next