Update List Item SPfx not working

Question

Below is the code, which is not working, shows below error

An unexpected 'PrimitiveValue' node was found when reading from the JSON reader. A 'StartObject' node was expected.

 const body: string = {
                    '__metadata': { 'type': 'SP.Data.InputsListItem' }, 'IsMapped': true
                };
    this.props.spHttpClient.post(`${this.props.siteUrl}/_api/web/lists/getbytitle('${this.props.listName}')/items(${item.Id})`,
      SPHttpClient.configurations.v1,
      {
        headers: {
          'Accept': 'application/json;odata=nometadata',
          'Content-type': 'application/json;odata=verbose',
          'odata-version': '',
          'IF-MATCH': '*',
          'X-HTTP-Method': 'MERGE'
        },
        body: JSON.stringify(body)
      });
  })
  .then((response: SPHttpClientResponse): void => {
    this.setState({
      status: `Item with ID: ${latestItemId} successfully updated`,
      items: []
    });
  }, (error: any): void => {
    this.setState({
      status: `Error updating item: ${error}`,
      items: []
    });

Answer 1

My test code for your reference.

let body: string = JSON.stringify({
      '__metadata': { 'type': 'SP.Data.InputsListItem' }, 'IsMapped': true
  });
  body = body.substring(1, body .length-1);
  body = '{' + body +'}';
  this.context.spHttpClient.post(`${this.context.pageContext.web.absoluteUrl}/_api/web/lists/getbytitle('Inputs')/items(1)`,
      SPHttpClient.configurations.v1,
      {
      headers: {
      'Accept': 'application/json;odata=nometadata',
      'Content-type': 'application/json;odata=verbose',
      'odata-version': '',
      'IF-MATCH': '*',
      'X-HTTP-Method': 'MERGE'
      },
      body: body
      })
      .then((response: SPHttpClientResponse): void => {
        console.log(response);
      }, (error: any): void => {
        console.log(error);
      });

Answer 2

You can do it without Rest with sp-pnp-js package:

import sp from "sp-pnp-js";

sp.web.lists.getByTitle(this.props.listName)
            .items.getById(item.Id).update({ IsMapped: true })