typeerror assignment to constant variable. in postman

TypeError: Assignment to Constant Variable in JavaScript

Last updated: Mar 2, 2024 Reading time · 3 min

# TypeError: Assignment to Constant Variable in JavaScript

The "Assignment to constant variable" error occurs when trying to reassign or redeclare a variable declared using the const keyword.

When a variable is declared using const , it cannot be reassigned or redeclared.

Here is an example of how the error occurs.

# Declare the variable using let instead of const

To solve the "TypeError: Assignment to constant variable" error, declare the variable using the let keyword instead of using const .

Variables declared using the let keyword can be reassigned.

We used the let keyword to declare the variable in the example.

Variables declared using let can be reassigned, as opposed to variables declared using const .

You can also use the var keyword in a similar way. However, using var in newer projects is discouraged.

# Pick a different name for the variable

Alternatively, you can declare a new variable using the const keyword and use a different name.

We declared a variable with a different name to resolve the issue.

The two variables no longer clash, so the "assignment to constant" variable error is no longer raised.

# Declaring a const variable with the same name in a different scope

You can also declare a const variable with the same name in a different scope, e.g. in a function or an if block.

The if statement and the function have different scopes, so we can declare a variable with the same name in all 3 scopes.

However, this prevents us from accessing the variable from the outer scope.

# The const keyword doesn't make objects immutable

Note that the const keyword prevents us from reassigning or redeclaring a variable, but it doesn't make objects or arrays immutable.

We declared an obj variable using the const keyword. The variable stores an object.

Notice that we are able to directly change the value of the name property even though the variable was declared using const .

The behavior is the same when working with arrays.

Even though we declared the arr variable using the const keyword, we are able to directly change the values of the array elements.

The const keyword prevents us from reassigning the variable, but it doesn't make objects and arrays immutable.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• SyntaxError: Unterminated string constant in JavaScript
• TypeError (intermediate value)(...) is not a function in JS

Borislav Hadzhiev

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Copyright © 2024 Borislav Hadzhiev

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TypeError: invalid assignment to const "x"

The JavaScript exception "invalid assignment to const" occurs when it was attempted to alter a constant value. JavaScript const declarations can't be re-assigned or redeclared.

What went wrong?

A constant is a value that cannot be altered by the program during normal execution. It cannot change through re-assignment, and it can't be redeclared. In JavaScript, constants are declared using the const keyword.

Invalid redeclaration

Assigning a value to the same constant name in the same block-scope will throw.

Fixing the error

There are multiple options to fix this error. Check what was intended to be achieved with the constant in question.

If you meant to declare another constant, pick another name and re-name. This constant name is already taken in this scope.

const, let or var?

Do not use const if you weren't meaning to declare a constant. Maybe you meant to declare a block-scoped variable with let or global variable with var .

Check if you are in the correct scope. Should this constant appear in this scope or was it meant to appear in a function, for example?

const and immutability

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered. This means that you can't mutate the value stored in a variable:

But you can mutate the properties in a variable:

Solve real coding problems

Postman – Save a response value into a variable

When sending a request with Postman , we can parse a value from the response and save it into a global variable or environment variable. This is useful when we need to use a value obtained from one request in other requests; for example, when we obtain an access token that needs to be included as the authorization header in other requests.

To keep the examples simple, I used REQ|RES (a mock REST API) for my requests and responses.

Parse response value into a global variable

Global variables are available across all Postman environments.

In this example, we want to save a token returned by a login request.

To parse the value of the “token” field into a global variable called “oauth_token”, click on the Tests tab and add the following JavaScript code:

The next time you execute the request, the token is saved into the “oauth_token” global variable.

To access the variable value, enter {{oauth_token}} . For example, in the following request we use the “oauth_token” variable in the authorization header.

Parse response value into an environment variable

Environment variables are only available in the environment you specify. Parsing a value into an environment variable is very similar to parsing a value into a global variable, which we explained above.

To parse the value of the “token” field into an environment variable called “oauth_token”, in the currently selected environment, click on the Tests tab and add the following JavaScript code:

Note: If you use the same name for an environment variable and a global variable, the environment variable takes precedence .

The next time you execute the request, the token is saved into a variable called “oauth_token” in the environment you have selected (“Development” in our case).

To see an environment variables and their values, click on the eye icon next to the environment name.

To access the variable value, enter {{oauth_token}} . For example, in the following request we use the “oauth_token” variable in the authorization header. You must select the correct environment (“Development” in our case) in order to access the variable value.

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4 thoughts on “Postman – Save a response value into a variable”

Simple and what i needed. Thank u r a life saver.

I’m glad this helped!

In insomnia it’s much easier

Interesting. I haven’t used that before.

Leave a Comment Cancel reply

Is it possible to assign the status of the test to any variable or constant?

My question : I want to see whether is it possible to assign the status of the response to any variable or constant? I want to have the if condition where if status is “201 created” then particular test cases should not run.

Thank you:)

The response code is available in script as pm.response.code , so you could do something like this:

Thank you very much @neilstudd . You are amazing.

JavaScript for Testers Part 1: JS Basics for Postman

Too Long; Didn't Read

README.FIRST

This write-up is a log of the author’s personal challenge to learn the bare minimum basics of JavaScript in a tight timeline for a specific purpose.

Please, take this piece as a “foundation building up reading” if you are someone like me who just want to take API testing and the automation of rigorous tasks a step further with Postman and JavaScript. The author logged the bare-minimum JS basics he thinks are needed for API testing scripting in Postman . As such, this article omitted a good number of important topics like forEach and while loops and try - catch flow etc. that are a must for those who want to learn JS seriously.

The target audience of this log is SQA engineers and testers who have prior knowledge of one or two programming languages and clearly know what they are doing to get what output.

Scripting Scopes in Postman: Pre-request Script and Tests

Just a humble revision to the scripting scopes in Postman is given below:

Pre-request Script : In this tab, we write the scripts that will be run before making an HTTP request. These scripts include setting up and assigning environment variables, any modification of the request body, etc.

Tests : Scripts written in this tab are run after an HTTP request ends. These scripts include different tests like looking for specific data in the response body, setting up environment variables for the next request/s, testing response codes and time, etc.

Response : Response body is shown in this window. We can select format options like JSON , HTML etc. from there.

Console : This tab prints the request headers, errors, and any JavaScript code outputs.

You need not to worry about running the collection. It is available in the cloud here .

For this tutorial, I used test APIs in DummyJSON .

Postman has a node.js runtime that gives us the ability to modify HTTP requests dynamically.

The scripts we write in the Pre-request and Tests tabs are run in the Postman sandbox.

The following resources are two must bookmarks for an API tester who uses Postman

Scripting in Postman

Postman JavaScript reference

Day 1: Variables

Variables are the fundamental building blocks of a program or script. We store values in them. A variable in JavaScript can be declared using either var , let or const keywords.

As I have a specific scope for writing test cases for Postman , I have chosen let and const for brevity after doing some R&D with a JS veteran and on the internet.

• Using let for variables that can be changed dynamically
• Using const for variables that should be untouched

In the following example, we will declare some JS variables, set and get Postman environment variables in the Pre-request Script and Tests tabs that are the fundamental building blocks to script in the API testing tool.

Example Request: DAY_1_All_Products_VARIABLES

Action : Submit

The following codes are self-explanatory:

Pre-request Script tab

Sample response

Please read the Day 1 section in my GitHub repository 7_Days_of_JS to learn more about JavaScript variables.

Day 2: Arrays

Arrays are indexed collections of the same or different types of objects. The behavior of arrays varies on the philosophy of programming languages. A JavaScript array can hold multiple types of objects in its indexes.

The basics of JS arrays are available here in my GitHub repository. The examples are well commented.

In the following Tests script, we will extract an array from a JSON response and append some arbitrary data in it.

Example Request : Day_2_All_Products_ARRAYS

Action : Send

Now look at the response JSON in the Body tab of the response window. We will extract the images array of the sixth item from it, which holds different images of a MacBook Pro .

Let’s parse the JSON, extract and print the expected array in the console

Click the small arrow sign beside (4) to expand the object for a more readable formatted view.

The array contains four URLs to a MacBook Pro laptop.

Now, we will push different types of objects into it and see the effect.

Notice, the length of sixthElementImages is increased from 4 to 7 as we pushed three new objects into it. Basic indexed operations are similar to other programming languages.

An interesting twist for geeks— a JS array is an object under-the-hood. PoC

Day 3: Objects

Objects, a key:value pair entity, play a vital role in JavaScript language that is used to store different types of data, variables, and functions for using them efficiently. In JavaScript, we can directly declare objects without creating a class.

Let’s declare a simple object.

Some features of an object to be noticed:

It is a common practice to declare an object with const for avoiding unexpected assignments. This will be elaborated on later.

Although key/property names can be written without quotations, it is mandatory to use them if the key contains any special character or words reserved for JS.

Here is a PoC why we use const for declaring an object

There are two more important fundamental concepts of deep and shallow copy of JavaScript objects discussed in my GitHub repository . The code is self-explanatory with necessary comments.

In the following example, we will convert a JS object into a JSON object and send the request body in a POST method to create an entity in the remote server.

Note: Keep in mind that JSON and JS objects are two different things.

Now paste this string into the POST request body, select raw radio button and select JSON from the right drop-down menu and finally, click the Beautify button to make the string more readable.

Example Request: Day_3_POST_Add_Product_OBJECTS

Action: Submit

JSON in Body tab after beautification

Day 4: Loops

A loop repeats the workflow of a code block for a limited or infinite time. There are four types of for loops in JavaScript

• Conventional
• for .. in loop
• for .. of loop
• forEach loop that calls a callback function to control the workflow

I will discuss forEach and while in a future article.

The syntax of the first three type of for loops are:

A rule of thumb to remember the for .. in vs for .. of is

• an in of for .. in returns the index of an object
• an of of for .. of returns the value or object of a collection of objects

Day 5: Flow

The basic execution flow of a JavaScript code is controlled by if else blocks.

The following code chunk written in the Tests block filters outs odd-number product ids that are divisible by 15 and 23

Example Request : Day_5_All_Products_FLOW

I will discuss try and catch in a future article.

Day 6: Functions

A function is a group of codes that we can reuse in a scope. This is the very first concept of code re-usability. The format of a function in JavaScript is given below

Let’s write a basic function in the Tests tab that will check if an input is a prime number or not.

Now, we will use this code block in a Postman request to filter out products that have a prime number price value.

Example Request : Day_6_All_Products_FUNCTIONS

Wrapping it Up

So far this was my 7-day journey to explore the tip of the JavaScript iceberg. In the next installment, I will come back with the basics of how to write test cases in Postman using JavaScript along with other necessary API testing tips and tricks.

I also wish to make Bangla video tutorials on API testing with JS and Postman in the future.

Until then, Happy Hacking !

About Author

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6. Introduction to variables and scripting

Variables in Postman

Previously in the Request Parameters section, we saw how using a variable saved us time and helped reduce redundant copy-paste of the request URL using the double curly brace syntax like this: . Remember, Postman allows you to save values as variables so that you can:

In this section, we will learn more about variables and introduce better practices that enables us to make dynamic requests.

Variable scopes

You can set variables that live at various scopes . Postman will resolve to the value at the nearest and narrowest scope. From broadest to narrowest, these scopes are global , collection , environment , data , and local .

If a variable with the same name is declared in two different scopes, the value stored in the variable with the narrowest scope will be used. For example, if there is a global variable named username and a local variable named username , the latter will be used when the request runs.

We will work with collection variables today, which live at the collection level and can be accessed anywhere inside the collection.

Settings variables programmatically

Scripting in postman.

Postman allows you to add automation and dynamic behaviors to your collections with scripting . Postman will automatically execute any provided scripts during two events in the request flow:

In this lesson, we will focus on writing scripts in the Tests tab, which are executed when a response comes back from an API.

The pm object

Postman has a helper object named pm that gives you access to data about your Postman environment, requests, variables and testing utilities. For example, you can access the JSON response body from an API with: pm.response.json() . You can also programmatically get collection variables like the value of baseUrl with: pm.collectionVariables.get(“baseUrl”) .

In addition to getting variables, you can also set them with: pm.collectionVariables.set("variableName", "variableValue") .

Task: Your first script

If you are new to JavaScript, here are some basics.

Logging data

In JS you can print data for a value to the console using this syntax:

In JS you can add comments to your code. These are skipped by the interpreter,so you can use them to explain things in your code.

Add a script to your request

Inside the Tests Editor , add this JS code to log the JSON response from the API:

Open the Postman Console in the lower left of the window:

Scroll to the bottom of the logs in the console. You will see the more recent request: POST https://library-api.poistmanlabs.com/books . The response data from the API is logged in the console because of the code in our Tests tab. You can expand the data by clicking on the small arrow to the left:

Task: Grab the new book id

Combining the power of variables and scripting gives you superpowers! Let’s explore how you can automatically set a value for a variable via scripting. Saving a value as a variable allows you to use it in other requests. Using a Test script, let’s grab the id of a newly added book and save it so we can use it in future requests.

Setting and getting collection variables

The pm object allows you to set and get collection variables. To set a collection variable, use the .set() method with two parameters: the variable name and the variable value: pm.collectionVariables.set("variableName", value) . To get a collection variable use the .get() method and specify the name of the variable: pm.collectionVariables.get("variableName") .

Local variables

We can also store local variables inside our Test script using JS. There are two ways to define a variable in JS:

const is for variables that won’t change value, whereas let allows you to reassign the value later.

Set the new book id as a variable

In the Tests tab of thet add a book request, replace the console.log() statement with this code:

If there is no Collection variable named id postman will create a new variable named id and assign the value.

Save and Send the request. When the 201 reponse comes back from the API, the test script will run and save the book’s id as a collection variable automatically.

View your collection variables by clicking on your Postmane Library API v2 collection, then the Variables tab. The id variable has been automatically assigned the id of your new book as its CURRENT VALUE .

You can now use `` anywhere in your collection to access this value.

Tips to resolve all your errors :

Trending Tags

Pin variables in Postman Flows

You can pin variables in Postman Flows so they keep their values across different loop iterations. When a variable in a Flow is pinned, its value persists through each iteration of the loop until a new value is explicitly set. Unpinned variables aren't retained after the iteration ends, or are reset at the beginning of the next iteration.

Unpinned variables offer flexibility for Flows that use dynamic data processing, so that each iteration can adapt based on new inputs or changes in the workflow context. Unpinning is useful in loops where each pass requires fresh data or where the variable's value is updated often.

Pin or unpin a variable in Postman Flows

To pin or unpin a variable, select the pin icon.

Pinned variable behavior in loops

A pinned variable defined outside of a loop retains its value in each iteration. If a pinned variable is both defined and used inside a loop, it will be destroyed and recreated in each iteration.

For example, this diagram shows an if loop with the pinned variable defined outside the loop:

The pinned variable retains its value in each iteration of the loop, so this loop will complete.

Conversely, this diagram shows an if loop with the pinned variable defined inside the loop:

The pinned variable is destroyed and recreated each time the loop iterates so the loop won't complete.

Last modified: 2024/03/15

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Mobile app development, nodejs typeerror: assignment to constant variable.

Published By: Divya Mahi

Published On: November 17, 2023

Published In: Development

Grasping and Fixing the 'NodeJS TypeError: Assignment to Constant Variable' Issue

Introduction.

Node.js, a powerful platform for building server-side applications, is not immune to errors and exceptions. Among the common issues developers encounter is the “NodeJS TypeError: Assignment to Constant Variable.” This error can be a source of frustration, especially for those new to JavaScript’s nuances in Node.js. In this comprehensive guide, we’ll explore what this error means, its typical causes, and how to effectively resolve it.

Understanding the Error

In Node.js, the “TypeError: Assignment to Constant Variable” occurs when there’s an attempt to reassign a value to a variable declared with the const keyword. In JavaScript, const is used to declare a variable that cannot be reassigned after its initial assignment. This error is a safeguard in the language to ensure the immutability of variables declared as constants.

Diving Deeper

This TypeError is part of JavaScript’s efforts to help developers write more predictable code. Immutable variables can prevent bugs that are hard to trace, as they ensure that once a value is set, it cannot be inadvertently changed. However, it’s important to distinguish between reassigning a variable and modifying an object’s properties. The latter is allowed even with variables declared with const.

Common Scenarios and Fixes

Example 1: reassigning a constant variable.

Javascript:

Fix: Use let if you need to reassign the variable.

Example 2: Modifying an Object's Properties

Fix: Modify the property instead of reassigning the object.

Example 3: Array Reassignment

Fix: Modify the array’s contents without reassigning it.

Example 4: Within a Function Scope

Fix: Declare a new variable or use let if reassignment is needed.

Example 5: In Loops

Fix: Use let for variables that change within loops.

Example 6: Constant Function Parameters

Fix: Avoid reassigning function parameters directly; use another variable.

Example 7: Constants in Conditional Blocks

Fix: Use let if the variable needs to change.

Example 8: Reassigning Properties of a Constant Object

Fix: Modify only the properties of the object.

Strategies to Prevent Errors

Understand const vs let: Familiarize yourself with the differences between const and let. Use const for variables that should not be reassigned and let for those that might change.

Code Reviews: Regular code reviews can catch these issues before they make it into production. Peer reviews encourage adherence to best practices.

Linter Usage: Tools like ESLint can automatically detect attempts to reassign constants. Incorporating a linter into your development process can prevent such errors.

Best Practices

Immutability where Possible: Favor immutability in your code to reduce side effects and bugs. Normally use const to declare variables, and use let only if you need to change their values later .

Descriptive Variable Names: Use clear and descriptive names for your variables. This practice makes it easier to understand when a variable should be immutable.

Keep Functions Pure: Avoid reassigning or modifying function arguments. Keeping functions pure (not causing side effects) leads to more predictable and testable code.

The “NodeJS TypeError: Assignment to Constant Variable” error, while common, is easily avoidable. By understanding JavaScript’s variable declaration nuances and adopting coding practices that embrace immutability, developers can write more robust and maintainable Node.js applications. Remember, consistent coding standards and thorough code reviews are your best defense against common errors like these.

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