Upload ContentVersion to SF via JS

You have to make sure that the VersionData has base64 encoded string for the file.

Add one more function to convertBlobtoBase64

export const convertBlobToBase64 = blob => new Promise((resolve, reject) => {
  const reader = new FileReader;
  reader.onerror = reject;
  reader.onload = () => {
     resolve(reader.result);
  };
  reader.readAsDataURL(blob);
});

Then modify your code to

let response = await 
 fetch('http://www.africau.edu/images/default/sample.pdf');
 let blob = await response.blob()
 .then(convertBlobToBase64)
 .then((result) => {
            fileContent = result;
            const base64Constant = 'base64,';
            const base64ImageValue =
                fileContent.indexOf(base64Constant) + base64Constant.length;
            this.base64Data = fileContent.substring(base64ImageValue);
            let cv = new sforce.SObject('ContentVersion');
            cv.Title = filename;
            cv.PathOnClient = filename;
            cv.VersionData  = blob;
            cv.VersionData = cv.VersionData.substring(dataStart);
            cv.ContentLocation = 'S';

            let result = await sforce.connection.create([cv]);
            console.log('result > ', result);
     });

You can then use the Base64 encoded string in versionData