Element implicitly has an "any" type because expression of type "string" can"t be used to index

🚀 Solving "Element implicitly has an 'any' type because expression of type 'string' can't be used to index" Error in TypeScript React Component

If you're trying to use TypeScript for your React project and encountering the following error:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{ train_1: boolean; train_2: boolean; train_3: boolean; train_4: boolean; }'.
  No index signature with a parameter of type 'string' was found on type '{ train_1: boolean; train_2: boolean; train_3: boolean; train_4: boolean; }'

Don't worry! You're not alone. This error usually occurs when you're trying to filter an array in your component and encounter issues related to indexing objects in TypeScript. Let's go through the code and find a solution together.

The Error in Context

Looking at the code snippet you provided, the error occurs in this line:

.filter(({ name }) => plotOptions[name]);

This line is attempting to filter an array using a property called name from plotOptions. However, TypeScript is flagging an error because it cannot infer the type and enforce that only correct keys can be used to index the plotOptions object.

Understanding the Code

To solve this error, let's understand the code.

Interfaces and Types

The component FiltrationPlots accepts an array of data through its props. It also has two interfaces defined:

interface IProps {
  data: any;
}

interface IState {
  [key: string]: plotTypes;
  plotOptions: plotTypes;
}

type plotTypes = {
  [key: string]: boolean;
  train_1: boolean;
  train_2: boolean;
  train_3: boolean;
  train_4: boolean;
};

• IProps defines the required prop data.

• IState is the component's state, which holds an object plotOptions of type plotTypes.

• plotTypes represents an object with keys as strings and boolean values. The specific keys are train_1, train_2, train_3, and train_4.

The Filtering Issue

In the render() method of the component, the filtering is triggered by this line:

const plotData: Array<trainInfo> = [...].filter(({ name }) => plotOptions[name]);

This line filters the plotData array by checking the name property against the plotOptions object.

The Solution

To fix the error, we need to provide TypeScript with the necessary information to correctly typecheck the indexing operation.

1. First, add an index signature to the plotTypes type:

type plotTypes = {
  [key: string]: boolean;
  train_1: boolean;
  train_2: boolean;
  train_3: boolean;
  train_4: boolean;
} & { [key: string]: boolean };

By adding { [key: string]: boolean } after the specific keys, TypeScript will allow any string keys to be used to index the type.

1. Update the IState interface to include the correct types:

interface IState {
  plotOptions: plotTypes;
}

Now, we've properly defined the state type to only include plotOptions, which is of type plotTypes.

Final Code

Here's the updated code for the component:

import React, { Component } from "react";
import createPlotlyComponent from "react-plotly.js/factory";
import Plotly from "plotly.js-basic-dist";
const Plot = createPlotlyComponent(Plotly);

interface IProps {
  data: any;
}

type plotTypes = {
  [key: string]: boolean;
  train_1: boolean;
  train_2: boolean;
  train_3: boolean;
  train_4: boolean;
} & { [key: string]: boolean };

interface IState {
  plotOptions: plotTypes;
}

interface trainInfo {
  name: string;
  x: Array<number>;
  y: Array<number>;
  type: string;
  mode: string;
}

class FiltrationPlots extends Component<IProps, IState> {
  readonly state = {
    plotOptions: {
      train_1: true,
      train_2: true,
      train_3: true,
      train_4: true
    }
  };

  render() {
    const { data } = this.props;
    const { plotOptions } = this.state;

    if (data.filtrationData) {
      const plotData: Array<trainInfo> = [
        // Plot data array goes here
      ].filter(({ name }) => plotOptions[name]);

      return (
        <Plot
          data={plotData}
          layout={{ width: 1000, height: 1000, title: "A Fancy Plot" }}
        />
      );
    } else {
      return <h1>No Data Loaded</h1>;
    }
  }
}

export default FiltrationPlots;

Conclusion

By providing TypeScript with the necessary type information and updating the interfaces, we were able to fix the error related to indexing objects.

Feel free to implement this solution in your React project and let us know if you face any further issues.

If you found this blog post helpful or have any questions, please leave a comment below. Happy coding! 😉