CORS（Cross-Origin Resource Sharing）是一种安全策略，用来限制不同域上的资源的交互。当使用Angular Http服务请求Express REST API时，由于域名不同，可能会出现CORS问题。为了解决这个问题，可以在Express API的响应头中设置Access-Control-Allow-Origin、Access-Control-Allow-Methods和Access-Control-Allow-Headers等属性，允许请求站点的跨域访问。示例代码如下：

Express代码：

const express = require('express')
const app = express()

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Methods", "GET, PUT, POST, DELETE");
  res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
  next();
});

app.get('/api/data', (req, res) => {
  res.send({message: 'Express API Response'})
})

app.listen(3000, () => {
  console.log('Express server started at http://localhost:3000')
})

Angular代码：

import { HttpClient, HttpHeaders } from '@angular/common/http';
import { Observable } from 'rxjs';
import { Injectable } from '@angular/core';

@Injectable()
export class DataService {

  constructor(private http: HttpClient) { }

  getData(): Observable {
    const headers = new HttpHeaders().set('Content-Type', 'application/json');
    return this.http.get('http://localhost:3000/api/data', { headers });
  }

}