yii\validators\RequiredValidator::clientValidateAttribute PHP Method

    public function clientValidateAttribute($model, $attribute, $view)
    {
        $options = [];
        if ($this->requiredValue !== null) {
            $options['message'] = Yii::$app->getI18n()->format($this->message, ['requiredValue' => $this->requiredValue], Yii::$app->language);
            $options['requiredValue'] = $this->requiredValue;
        } else {
            $options['message'] = $this->message;
        }
        if ($this->strict) {
            $options['strict'] = 1;
        }
        $options['message'] = Yii::$app->getI18n()->format($options['message'], ['attribute' => $model->getAttributeLabel($attribute)], Yii::$app->language);
        ValidationAsset::register($view);
        return 'yii.validation.required(value, messages, ' . json_encode($options, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE) . ');';
    }