ReactJS - How to restrict data type for different kind of data
Tags: react, javascript
Published: 2020-07-05 16:46:31

## Introduction
Javascript is not a strict type language. We can use a variable to save any kind of data. Whether its a string or integer or boolean or object. Well, this gives aa flexibility in using the variabales. But, it brings some complexity of making sure that the data is of certain expected type only.

For example, you are showing data of students as:

- Id
- Name
- Image
- Age

Example reactjs component:

```
import React, { Component } from "react";

const Student = ({ name, age, img }) => {
  return (
    <article>
      <img src={img} alt="student" />
      <h5>name : {name}</h5>
      <h5>age : {age}</h5>
    </article>
  );
};

class StudentList extends Component {
  state = {
    students: [
      {
        id: 1,
        img: "some img path",
        name: "Raman",
        age: 21
      },
      {
        id: 2,
        img: "Some image path",
        name: "Rahul",
        age: 22
      }
    ]
  };
  render() {
    return (
      <section>
        {this.state.students.map(student => (
          <Student
            key={student.id}
            img={student.img}
            name={student.name}
            age={student.age}
          />
        ))}
      </section>
    );
  }
}

class App extends Component {
  render() {
    return <StudentList />;
  }
}

export default App;
```

If you messed up somewhere in your data. You might ends up showing age in place of name, or name in place of age. Since, there will be no error or warning unless you are doing some special operation with that variable.

Lets discuss abaout making our type strict.

## Prop Types
There is a npm module for this purpose: prop-types. To install this module:

```
npm install prop-types
```

## How to use
Lets see the modified component:

```
import React, { Component } from "react";
import PropTypes from "prop-types";

const Student = ({ name, age, img }) => {
  return (
    <article>
      <img src={img} alt="student" />
      <h5>name : {name}</h5>
      <h5>age : {age}</h5>
    </article>
  );
};
Student.propTypes = {
  name: PropTypes.string,
  age: PropTypes.number,
  img: PropTypes.string
};

class StudentList extends Component {
  state = {
    students: [
      {
        id: 1,
        img: "some img path",
        name: "Raman",
        age: 21
      },
      {
        id: 2,
        img: "Some image path",
        name: "Rahul",
        age: 22
      }
    ]
  };
  render() {
    return (
      <section>
        {this.state.students.map(student => (
          <Student
            key={student.id}
            img={student.img}
            name={student.name}
            age={student.age}
          />
        ))}
      </section>
    );
  }
}

class App extends Component {
  render() {
    return <StudentList />;
  }
}

export default App;
```

Notice the usage of propTypes as:

```
Student.propTypes
```

We are declaring the data types of certain type of data. And, if we try to provide any other kind of data to such variables, react will complain. It will throw errors. And you will come to know the mistakes.

## How to restrict data to be Required (Not Null)
In many scenarios we would require that the data not be null. With prop-types you can specify or restrict with some attributes that this particular data must be present.

Lets see how to do this:

```
Student.propTypes = {
  name: PropTypes.string.isRequired,
  age: PropTypes.number.isRequired,
  img: PropTypes.string
};
```

Note: Above code will work if you are receiving your values as individual fields. If you are receiving the object, above code will not work. For objects, we have a slightly different way:

```
Student.propTypes = {
  student: PropTypes.shape({
    name: PropTypes.string.isRequired,
    age: PropTypes.number.isRequired,
    img: PropTypes.string
  })
};
```

In above code, we have decided that name and age must be present. If our data does not have these fields, then we will get errors.

Also see [How to create ReactJS Components](https://www.gyanblog.com/gyan/reactjs-how-create-components/)

## How to configure default properties
In many cases, our data does not have some of the attributes. Example: Image is not there. And we would want to declare some default values. It means, if our data does not have any value for particular attribute, please use this default value.

```
Student.defaultProps = {
  img: "some default image path"
};
```

prop-types does not support Objects for default values.

Originally posted at: [ReactJS - How to restrict data type for different kind of data](https://www.gyanblog.com/gyan/reactjs-how-restrict-data-types-data/)