TypeScript Object to FormData, with support for nested objects, arrays and File objects.

object-to-formdata.ts

  static convertModelToFormData(model: any, form: FormData = null, namespace = ''): FormData {
    let formData = form || new FormData();
    for (let propertyName in model) {
      if (!model.hasOwnProperty(propertyName) || model[propertyName] == undefined) continue;
      let formKey = namespace ? `${namespace}[${propertyName}]` : propertyName;
      if (model[propertyName] instanceof Date) {        
        formData.append(formKey, this.dateTimeToString(model[propertyName]));
      }
      else if (model[propertyName] instanceof Array) {
        model[propertyName].forEach((element, index) => {
          if (typeof element != 'object')
            formData.append(`${formKey}[]`, element);
          else {
            const tempFormKey = `${formKey}[${index}]`;
            this.convertModelToFormData(element, formData, tempFormKey);
          }
        });
      }
      else if (typeof model[propertyName] === 'object' && !(model[propertyName] instanceof File)) {        
        this.convertModelToFormData(model[propertyName], formData, formKey);
      }
      else {        
        formData.append(formKey, model[propertyName].toString());
      }
    }
    return formData;
  }

First of all, namespace is a reserved keyword in TypeScript(HandBook and Microsoft/TypeScript@GitHub).
Also, it seems that you forgot to deal with File since the last else will append "[object File]" to the formData