How to get Basic Constraints in a certificate by JS

everyone
I’m trying use JavaScript to parse a PEM format certificate now. I’m expecting to get the
Basic Constraint value to distinguish whether a certificate the user uploaded is a CA.
The Basic Constraint value in the certificate is as follows:

My coding environment is Vue2 and jsrsasign v11.1.0 and nodejs v18.9.0.
My code is bellow.

<template>
  <div class="parseCert">
    <el-upload
      class="uploadFile"
      ref="upload"
      drag
      action="aa"
      :on-change="onChange"
      :auto-upload="false">
      <i class="el-icon-upload"></i>
      <div class="el-upload__text">Drag cert file to here</div>
    </el-upload>
  </div>
</template>

<script>
  import { X509 } from 'jsrsasign'
  export default {
    data () {
      return {
      }
    },
    methods: {
      onChange (file) {
        let reader = new FileReader()

        let cert = new X509()

        reader.onload = function () {

          cert.readCertPEM(reader.result)
          cert.getSubject().array.forEach(e => {
            console.log('ds: ' + e[0].ds + ', type: ' + e[0].type + ', value: ' + e[0].value)
            }
          )
          // Here is I try to get the basicConstraints
          // But it seems that it is nothing can be get
          const basicConstraints = cert.getExtBasicConstraints() 
          console.log('Basic Constraints: ' + basicConstraints)
          console.log(basicConstraints)
          for (const key in basicConstraints) {
            if (Object.prototype.hasOwnProperty.call(basicConstraints, key)) {
              const value = basicConstraints[key];
              console.log(`Key: ${key}, Value: ${value}`);
            }
          }
        }

        reader.readAsText(file.raw)
      }
    }
  }
</script>

<style>
</style>

My code can’t get the Basic Constraint value and I have no idea how to fix.
Am I using the jsrsasign module by mistake?
There is any better or right way to achieve my goal?