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Expected an assignment or function call and instead saw an expression

Reactjs guru.

• February 10, 2024

The error message “ Expected an assignment or function call and instead saw an expression ” in React.js typically arises due to a missing return statement within a function, where a value is expected to be returned but isn’t.

Table of Contents

Understanding the Error:

The error message “Expected an Assignment or Function Call and Instead Saw an Expression” typically occurs in React.js when a function or callback does not return a value as expected. This can happen when we use methods like map() without properly returning a value from the callback function.

Causes of the Error:

Forgetting to use a return statement in a function or callback can lead to this error. This often occurs when we use array methods map() where a return value is expected from the callback function.

Solving the Error:

There are two main approaches to solving the “Expected an Assignment or Function Call and Instead Saw an Expression” error in React.js: using explicit and implicit returns.

1. Using Explicit Return:

Explicitly using a return statement ensures that functions or callbacks return the expected values. This is especially important when we use array methods like map() .

2. Using Implicit Return:

Using an implicit return with arrow functions provides a concise and elegant solution. This shorthand syntax is suitable for simple functional components and ensures that functions implicitly return values without using the return keyword.

Conclusion:

 The “Expected an Assignment or Function Call and Instead Saw an Expression” error in React.js occurs when functions or callbacks fail to return values as expected, commonly due to incomplete definitions or incorrect usage of array methods. To fix this error, we can use explicit returns and implicit returns.

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How to use the forEach() method in React

Last updated: Apr 7, 2024 Reading time · 3 min

# Using the forEach() method in React

The forEach() method can be used to iterate over an array outside of your JSX code in React.

If you need to iterate over an array and render its elements directly in your JSX code, use the map() method instead.

The Array.forEach() method can be used when you need to call a function for every element in an array.

The forEach() method calls the provided function with each element in the array but returns undefined .

Using it directly in our JSX code wouldn't make sense because we need to return JSX elements and not undefined .

# Using forEach() to push JSX elements to an array in React

You could use the forEach() method to:

• Iterate over an array.
• Push JSX elements into a new array.
• Render the JSX elements.

Instead of directly rendering the object's values, we push the JSX markup for each object into a results array.

I've also written a detailed guide on how to loop through an object in React .

# Use the Array.map() method to render an array's elements

To iterate over an array and render its elements directly in your JSX code, use the Array.map() method.

The function we passed to the Array.map() method gets called for each element in the array.

You can also use the Array.map() method to render a nested array .

# Using a for...of loop instead of forEach()

You can also use the for...of loop in a similar way to how we used the forEach() method.

The for...of loop can also be used to iterate over an array of objects .

The break keyword cannot be used in the forEach() method, but it is supported in the for...of loop.

I've also written an article on how to sort an array of objects .

If you need to filter an array of objects in React, click on the following article .

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

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Expected an assignment or function call and instead saw an expression in ReactJS

In React.js we create components, inside these components , there are functions that we export and then use inside another component by importing it. Sometimes when you try to render that component or use that component as a tag inside another component, It throws an error “React: Expected an assignment or function call and instead saw an expression”.

Let’s see an example by creating a simple React application that throws the same error.

Steps to create a simple React application that just shows the heading “Geeks For Geeks”:

Step 1: Open the terminal and type the following command to create a react app.

change the <folder-name> to whatever you want, and press enter.

Step 2: Open the folder inside the code editor, and delete all the files inside the “src” folder except the index.js file.

Step 3: Delete all the files inside the public folder. Don’t worry we will create these files later according to our needs.

Project Structure: We have a simple project structure, we have a package.json & package-lock.json file which contains the details about the module installed inside this project. Then we have node_modules which contains the actual module and an “index.js” file inside “src”, our main server file.

Step 3: Create a file “index.html” inside the “public” folder and write the following code.

This is a basic HTML code, inside we have an “id” root when you open the “index.js” file you will see that the react mount everything to this id everything we do inside our components will show here.  

Make sure that your “index.js ” looks like this:

Step 4: By default “index.js” mount the “App.js” component which we just deleted, so let’s create “App.js” components. Components in React.js are nothing but a function that returns HTML tags. Let’s make a function that contains a heading.

In React components is nothing but a Javascript file that can contain a function, and then inside the function, we can write the HTML code, but in React.js the function should return something so we have to use a return statement, but in Javascript, the return can only have one-liner statements but in our case, we are passing many statements.

React expects it to be a valid Javascript function, if it is not then for React it is not a function, so it says “React: Expected an assignment or function call and instead saw an expression”, because It expects a function call and the function which doesn’t return is only an expression to it.

The solution is to use the return keyword and wrap all statements inside a bracket so that they become a one-liner statement for Javascript.

Complete Code:

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Fix- expected an assignment or function call and instead saw an expression

If you are getting expected an assignment or function call and instead saw an expression error, Here is a simplified version that gives the same warning:

This throws the error Expected an assignment or function call and instead saw an expression .

This means that you have an expression but do not assign the result to any variable. jshint doesn’t care about what the actual expression is or that there are side effects. Even though you assign something inside the expression, you are still ignoring the expression’s result.

There is another error by jslint if you care about it:

Unexpected assignment expression

This warns you that you may want to use == instead of = inside logical expressions. It’s a standard error. Therefore you are discouraged from using logical expression assignments (even though it is precisely what you want here).

Jshint/Jslint do not like misuse of shortcut evaluation of logical operator as a replacement for if statements. It assumes that if the result of an expression is not used, it probably shouldn’t be an expression.

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why do i get warning "expected an assignment or function call and instead saw an expression?"

when i type code:

I get the message “expected an assignment…..” Why? Also if you miss out the semi-colon at the end the warning alerts you to this also, but there is inconsistency in the placement of the semi-colon eg the course only introduces it at the prompts - never mentioned up until then. know you get the javascript semi-colon insertion but feel ignoring it then suddenly expecting it as indicated via the warning is inconsistent and confusing.

Answer 505ae9c0e763ee000202ae85

These are assignments :

The stuff on the right-hand side of an assignment (everything after the = ) is called an expression . Usually expressions don’t stand alone. The warning is just there so you don’t accidentally forget the variable (the assignment’s left-hand side) that you would normally assign that expression’s value to.

A function call is something like console.log("hi!") and is also an expression , but a rather special one – it is frequently found on its own on a line.

Semicolons can be omitted in some cases, but generally it’s good to get used to put them wherever the yellow warning sign says that a semicolon is missing. You can omit them later when you have understood exactly under which circumstances they can be omitted. Also, if removing a semicolon does not produce a warning, then the semicolon probably doesn’t belong there – semicolons in the wrong places make nasty bugs that are hard to find.

Thanks for the precise answer. Now I understand it’s a warning as opposed to an error. I see what you mean too about semi-colons producing annoying bugs, but with this in mind then the course should start out by introducing the semi-colons in the correct places e.g., during the very first questions on getting length. The first exercises are on the command line so you don’t get warnings , so I think for consistency and accuracy the semi-colon should be introduced from the start.

I noticed this as well, I would love to see something raised at the start to clarify the when’s and when not’s of semi-colons.

wow, that’s incredibly confusing: I’m on the first few lessons, knowing nothing about javascript and I’m supposed to know how to declare a variable??? Assignments are not explained in the first lessons…

Nobody expects you to understand the technical terms behind the code editor’s warnings right away. Closely following the instructions is the only requirement for finishing the exercises.

Answer 512e95ea07c83f1a3f001f2c

I had to play around with it for a while, but this is what you have to do: ;”cake”.length*9

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Expected an assignment or function call and instead saw an expression

When do i get this error.

The "Expected an assignment or function call and instead saw an expression" error is thrown when JSLint, JSHint or ESLint encounters an expression with no effect . In the following example we have a conditional expression that will evaluate to true but has no other effect on the program:

Why do I get this error?

This error is raised to highlight a piece of useless and unnecessary code . The code will work as expected but since a lone floating expression has no effect on anything there is no point in it being there at all.

In general you would expect to see a statement which has an effect, such as assigning a value to a variable or invoking a function:

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax . The identifier of this warning is W030 . This means you can tell JSHint to not issue this warning with the /*jshint -W030 */ directive.

In ESLint the rule that generates this warning is named no-unused-expressions . You can disable it by setting it to 0 , or enable it by setting it to 1 .

About the author

This article was written by James Allardice, Software engineer at Tesco and orangejellyfish in London. Passionate about React, Node and writing clean and maintainable JavaScript. Uses linters (currently ESLint) every day to help achieve this.

Follow me on Twitter  or  Google+

This project is supported by orangejellyfish , a London-based consultancy with a passion for JavaScript. All article content is available on GitHub under the Creative Commons Attribution-ShareAlike 3.0 Unported licence.

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DMnm6KBGkM3gQJixrQXQ6MVB9u6Fhcu7rr Optional chaining in TypeScript not supported by SonarJS Our developers are doing some TypeScript development, and are using the optional chaining ? feature. SonarJS is flagging the uses of this feature as a bug, Expected an assignment or function call and instead saw an expression. via the typescript:S905 rule: This has been reported numerous times: And the commonly linked GitHub issue is closed: S905: FP with optional chaining · Issue #1930 · SonarSource/SonarJS · GitHub We are using the latest available version of SonarJS, but are still experiencing this issue. Is this issue still being worked on? If so, what is the status, and is there an ETA? Currently the workaround is to manually resolve all instances of this bug in SonarQube, which is tedious to say the least. Another option is to entirely disable the typescript:S905 rule, which I don’t like. I apologize for opening yet another thread on this topic, but it’s unclear to me what the status of this issue is based on the other topics. Our toolchain: SonarQube 7.9.4 Enterprise SonarScanner MSBuild 4.7.1.2311 SonarJS 6.2.2, which is the latest available in the Marketplace TypeScript 3.7 As a followup, I’ve done some testing and this is definitely an issue specific to using the optional chaining ? feature. For example, in each of the two screenshots below you can see two lines of code, one using optional chaining ? , the other without it, but they are otherwise identical. The code with the chaining is flagged with the typescript:S905 rule. Hi @zapp-dennis , We will support optional chaining with SonarJS in SonarQube 8.5 based on our development team here , which will be released very soon. Keep on eye on the What’s new page and look for the latest download here . I also highly suggest you sign up for our announcement email on this page as well: Sonar Community - Home of Clean Code Please also note this: Since SonarTS v2.0, TypeScript analysis is performed by SonarJS analyzer v6.0 or later. No TypeScript analysis is performed by SonarTS.) Thank you for the response! Do you happen to know when the next SonarQube LTS will be released, which I’m assuming would include support for this feature? We’d like to avoid running the interim releases from a supportability standpoint. Yes, SonarQube 8.x LTS will most likely be released in early 2021, which should also support TypeScript 4, so yes, we will support optional chaining definitely in SonarQube 8.x LTS. Here’s our Roadmap page to give you an outlook on the horizon: https://www.sonarqube.org/roadmap/ This topic was automatically closed 7 days after the last reply. New replies are no longer allowed. Navigation Menu Search code, repositories, users, issues, pull requests..., provide feedback. We read every piece of feedback, and take your input very seriously. Saved searches Use saved searches to filter your results more quickly. To see all available qualifiers, see our documentation . Notifications You must be signed in to change notification settings Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community. By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement . We’ll occasionally send you account related emails. Already on GitHub? Sign in to your account Expected an assignment or function call and instead saw an expression #2866 asaadawey commented Dec 10, 2020 and my problem is not listed. : true, "parser": "@typescript-eslint/parser", "plugins": ["@typescript-eslint"], "extends": [ "eslint:recommended", "plugin:@typescript-eslint/eslint-recommended", "plugin:@typescript-eslint/recommended", "prettier/@typescript-eslint", // Uses eslint-config-prettier to disable ESLint rules from @typescript-eslint/eslint-plugin that would conflict with prettier "plugin:prettier/recommended" ], "overrides": [ { "files": ["*.tsx", "*.ts"], "rules": { "no-unused-expressions": "off", "@typescript-eslint/no-unused-expressions": ["off"] } } ], "rules": { "no-ternary": "off", "no-unused-expressions": "off", "@typescript-eslint/no-unused-expressions": ["off"] } } then(({ data }: FetchAllVehiclesQueryHookResult) => { //135 data?.vehicles.forEach((item) => { ..... //136 version `4.9.1 The text was updated successfully, but these errors were encountered: bradzacher commented Dec 10, 2020 • edited Loading You've skipped a lot of sections in the template. The issue template exists so that you provide all the necessary information up front to help with investigation Could you provide an isolated repro? What does the code actually look like? Also what is the actual error you're seeing? What line is it? What's throwing it? Sorry, something went wrong. KhodeN commented Jan 12, 2021 • edited Loading The error appears for expressions like bradzacher commented Jan 12, 2021 Going to need more information. Please file a new issue. No branches or pull requests Get the Reddit app The (unofficial) React.js subreddit for all things React! Expected an assignment or function call and instead saw an expression no-unused-expressions I'm really new to React and I'm getting this error. This is my code, I have no clue what is wrong. By continuing, you agree to our User Agreement and acknowledge that you understand the Privacy Policy . Enter the 6-digit code from your authenticator app You’ve set up two-factor authentication for this account. 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Please be sure to answer the question.Provide details and share your research! But avoid …. Asking for help, clarification, or responding to other answers. (React) Expected an assignment or function call and instead saw an The code for this article is available on GitHub. We solved the issue in our map() method by explicitly returning. This is necessary because the Array.map () method returns an array containing all of the values that were returned from the callback function we passed to it. Note that when you return from a nested function, you aren't also ... Expected an assignment or function call and instead saw an expression React tips & tutorials delivered to your inbox. Don't miss out on the latest insights, tutorials, and updates from the world of ReactJs! Our newsletter delivers valuable content directly to your inbox, keeping you informed and inspired. How to use the forEach() method in React The forEach() method calls the provided function with each element in the array but returns undefined. Using it directly in our JSX code wouldn't make sense because we need to return JSX elements and not undefined. # Using forEach() to push JSX elements to an array in React. You could use the forEach() method to: Iterate over an array. Expected an assignment or function call and instead saw an expression Step 1: Open the terminal and type the following command to create a react app. npx create-react-app <folder-name>. change the <folder-name> to whatever you want, and press enter. Step 2: Open the folder inside the code editor, and delete all the files inside the "src" folder except the index.js file. Step 3: Delete all the files inside the ... SonarLint VSCode extension warning when using optional chaining Operating system: Windows 10 IDE name and flavor/env: Visual Studio Code / TypeScript SonarLint plugin version: 1.22.0 / jdk1.8.0_251 As the title says, an optional chaining in forEach() gets me a warning message: "Exp… Array.prototype.forEach() The forEach() method is an iterative method. It calls a provided callbackFn function once for each element in an array in ascending-index order. Unlike map(), forEach() always returns undefined and is not chainable. The typical use case is to execute side effects at the end of a chain. Read the iterative methods section for more information ... reactjs Expected an assignment or function call and instead saw an expression no-unused-expressions (lines 14/15) 0 Line 9: Expected an assignment or function call and instead saw an expression no-unused-expressions Fix expected an assignment or function call and instead saw an expression Expected an assignment or function call and instead saw an expression. This means that you have an expression but do not assign the result to any variable. jshint doesn't care about what the actual expression is or that there are side effects. Even though you assign something inside the expression, you are still ignoring the expression's ... Got an error saying expected an assignment or function call: and If you have follow up questions, please reply to the existing post. No need to create a whole new post for basically the same problem. And the issue is that you aren't actually using the data from the API for data1 and data2.res.data contains the data from your API and you aren't assigning that anywhere in your state. why do i get warning "expected an assignment or function call and The warning is just there so you don't accidentally forget the variable (the assignment's left-hand side) that you would normally assign that expression's value to. A function call is something like console.log("hi!") and is also an expression, but a rather special one - it is frequently found on its own on a line. JSLint Error Explanations In general you would expect to see a statement which has an effect, such as assigning a value to a variable or invoking a function: var x = 1; x = 2; // Assignment instead of unused expression. In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W030 . Optional chaining in TypeScript not supported by SonarJS SonarJS is flagging the uses of this feature as a bug, Expected an assignment or function call and instead saw an expression. via the typescript:S905 rule: optional-chaining 817×107 7.06 KB. This has been reported numerous times: Typescript 3.7.2 Null conditional operator on a function call is giving a false positive SonarCloud. Javascript: Expected an assignment or function call and instead saw an It's quite self-explanatory. The || operator produces an expression, not a statement (assignment) or function call. Your code should work if you make it an assignment: iterator = iterator || _.identity; answered May 17, 2016 at 18:25. Scimonster. useEffect error: "Expected an assignment or function call and instead useEffect error: "Expected an assignment or function call and instead saw an expression". I have a React app that utilizes GSAP, which is called using a useEffect hook. It was working fine, but I recently upgraded to React 2.1.8 and 'ran npm run build' and issue below appeared... Expected an assignment or function call and instead saw an ... [T] I have tried restarting my IDE and the issue persists. [T ] I have updated to the latest version of the packages. [T ] I have read the FAQ and my problem is not listed. { "root": true, "parser"... Expected an assignment or function call and instead saw an ... PowerShell is a cross-platform (Windows, Linux, and macOS) automation tool and configuration framework optimized for dealing with structured data (e.g. JSON, CSV, XML, etc.), REST APIs, and object models. React 1. I'm new with reactjs and I got a little problem. I'm getting an error: Expect an assignment or function call. Is referring to the User function, but I do call that function below when I'm creating a new object. How can I fix this? let users = []; function User(name, surname, age, email, password){. this.name = name; React Expected an assignment or function call and instead saw an Please I'm new to programming and don't even know where to start this assignment is overwhelming. Can someone please help me, please. The program should read all given files ... WebPack React Eslint error in JSON: Expected an assignment or function If every function is continuous if and only if its projections are continuous, can we conclude the codomain has the product topology? An integral using Mathematica or otherwise Sticker on caption phone says that using the captions can be illegal. essay homework paper phd project resume review speech thesis