TypeError: fields.flat code example

Example: fields.flat is not a function

// Patched into :
static normalizeFields(...fields) {
    return fields
      .reduce((acc, val) => acc.concat(val), [])
      .map(field =>
        this.normalizeField(
          field && field.name,
          field && field.value,
          field && typeof field.inline === 'boolean' ? field.inline : false,
        ),
      );
  }