React.js - Syntax error: this is a reserved word in render() function

Question

I'm stuck on a error for the reserved keyword "this". In my React Component below shows me passing in a state from a my main component "App.js" to my "RecipeList.js" component to then map the data and render each RecipeItem Component. I just don't understand why I get this error

React.js - Syntax error: this is a reserved word

The error is called in RecipeList inside the render return method; If anybody could help that would great!

Thanks

App.js

//main imports
import React, { Component } from 'react';

//helper imports
import {Button} from 'reactstrap'
import RecipeItem from './components/RecipeItem';
import RecipeList from './components/RecipeList';
import './App.css';

const recipes = [
  {
    recipeName: 'Hamburger',
    ingrediants: 'ground meat, seasoning'
  },
  {
    recipeName: 'Crab Legs',
    ingrediants: 'crab, Ole Bay seasoning,'
  }
];

class App extends Component {
  constructor(props){
    super(props);
    this.state = {
      recipes
    };
  }

  render() {
    return (
      <div className="App">
        <div className = "container-fluid">
          <h2>Recipe Box</h2>
          <div>
            <RecipeList recipes = {this.state.recipes}/>
          </div>
        </div>
        <div className = "AddRecipe">
          <Button>Add Recipe</Button>
        </div>

      </div>
    );
  }
}

export default App;

RecipeLists.js

import React, {Component} from 'react';
import _ from 'lodash';
import RecipeItem from './RecipeItem';


class RecipeList extends Component {

    renderRecipeItems() {
      return _.map(this.props.recipes, recipeItem => <RecipeItem key = {i} {...recipes} />);
    }

    render() {
      return (
        { this.renderRecipeItems() }
      );
    }
}

export default RecipeList

nem035 nem035 · Accepted Answer · 2017-06-30T15:15:45

All the solutions given here are correct.

The easiest change is to just wrap your function call in a JSX element:

return (
  <div>
    { this.renderRecipeItems() }
  </div>
)

However, none of the answers are (correctly) telling you why the code was breaking in the first place.

For the sake of an easier example, let's simplify your code a bit

// let's simplify this
return (
  { this.renderRecipeItems() }
)

such that the meaning and behavior are still the same. (remove parenths and move curlies):

// into this
return {
  this.renderRecipeItems()
};

What this code does is it contains a block, denoted by {}, within which you're trying to invoke a function.

Because of the return statement, the block {} is treated like an object literal

An object literal is a list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}).

which expects either a: b or a (shorthand) syntax for it's property-value pairs.

// valid object
return {
  prop: 5
}

// also valid object
const prop = 5;
return {
  prop
}

However, you're passing a function call instead, which is invalid.

return {
  this.renderRecipeItems() // There's no property:value pair here
}

When going through this code, the engine assumes it will read an object-literal. When it reaches the this., it notices that . is not a valid character for a property name (unless you were wrapping it in a string - see bellow) and the execution breaks here.

function test() {
  return {
    this.whatever()
    //  ^ this is invalid object-literal syntax
  }
}

test();

For demonstration, if you wrapped your function call in quotes, code would accept the . as part of a property name and would break now because a property value is not provided:

function test() {
  return {
    'this.whatever()' // <-- missing the value so the `}` bellow is an unexpected token
  }
}

test();

If you remove the return statement, the code wouldn't break because that would then just be a function call within a block:

function test() {
  /* return */ {
    console.log('this is valid')
  }
}

test();

Now, an additional issue is that it's not the JS engine that is compiling your code but it's babel, which is why you get the this is a reserved word error instead of Uncaught SyntaxError: Unexpected token ..

The reason is that JSX doesn't accept reserved words from the JavaScript language such as class and this. If you remove this, you can see that the reasoning above still applies - babel tries to parse the code as an object literal that has a property, but no value, meaning babel sees this:

return {
  'renderRecipeItems()' // <-- notice the quotes. Babel throws the unexpected token error
}

React.js - Syntax error: this is a reserved word in render() function

3 Answers