Magento 2 how to show configurable options stock availability on product page

addtocart.phtml

<?php
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
// @codingStandardsIgnoreFile
/** @var $block \Magento\Catalog\Block\Product\View */
?>
<?php $_product = $block->getProduct(); ?>
<?php $buttonTitle = __('Add to Cart'); ?>
<?php if ($_product->isSaleable()): ?>
<div class="box-tocart">
<div class="fieldset">
<?php if ($block->shouldRenderQuantity()):
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$StockState = $objectManager->get('\Magento\CatalogInventory\Api\StockStateInterface');?>
<div class="field qty">
<label class="label" for="qty"><span><?= /* @escapeNotVerified */ __('Qty') ?></span></label>
<div class="control">
<?php if($_product->getTypeId()=="simple") { ?>
    <select class="input-text qty" name="qty" id="qty" data-validate="<?= $block->escapeHtml(json_encode($block->getQuantityValidators())) ?>">
<?php 
$availableQty= $StockState->getStockQty($_product->getId(), $_product->getStore()->getWebsiteId());
for($repli=1;$repli<=$availableQty;$repli++)
echo "<option value='$repli'>$repli</option>";
?>
</select>
<?php } else { 
$usedProducts = $_product->getTypeInstance()->getUsedProducts($_product);
$productStockd=$objectManager->get('Magento\CatalogInventory\Api\StockRegistryInterface');
$finalconfigqty=[];
foreach ($usedProducts as $child)
{$childid=$child->getId();
$productStockObj = $productStockd->getStockItem($childid);
$availableQty= round($productStockObj->getData('qty')); 
$finalconfigqty['c'.$childid]=$availableQty;
}
echo '<script>
var fichoo='.
json_encode($finalconfigqty).
'</script>';
echo '<select class="input-text qty fichoo" name="qty" id="qty"><option value=1>1</option></select>'; 
?> 
<!--input type="number" name="qty" id="qty" value="<?= $block->getProductDefaultQty() * 1 ?>" title="<?= __('Qty') ?>" class="input-text qty"
data-validate="<?= $block->escapeHtml(json_encode($block->getQuantityValidators())) ?>"
/--> <?php } ?>
</div>
</div>
<?php endif; ?>
<div class="actions">
<button type="submit"
title="<?= /* @escapeNotVerified */ $buttonTitle ?>"
class="action primary tocart"
id="product-addtocart-button">
<span><?= /* @escapeNotVerified */ $buttonTitle ?></span>
</button>
<?= $block->getChildHtml('', true) ?>
</div>
</div>
</div>
<?php endif; ?>
<?php if ($block->isRedirectToCartEnabled()) : ?>
<script type="text/x-magento-init">
{
"#product_addtocart_form": {
"Magento_Catalog/product/view/validation": {
"radioCheckboxClosest": ".nested"
}
}
}
</script>
<?php else : ?>
<script type="text/x-magento-init">
{
"#product_addtocart_form": {
"Magento_Catalog/js/validate-product": {}
}
}
</script>
<?php endif; ?>

Another important file is Magento_ConfigurableProduct/web/js/configurable.js

modified function _getSimpleProductId and added function _dofichoo

/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
/**
 * @api
 */
define([
    'jquery',
    'underscore',
    'mage/template',
    'mage/translate',
    'priceUtils',
    'priceBox',
    'jquery/ui',
    'jquery/jquery.parsequery'
], function ($, _, mageTemplate, $t, priceUtils) {
    'use strict';

    $.widget('mage.configurable', {
        options: {
            superSelector: '.super-attribute-select',
            selectSimpleProduct: '[name="selected_configurable_option"]',
            priceHolderSelector: '.price-box',
            spConfig: {},
            state: {},
            priceFormat: {},
            optionTemplate: '<%- data.label %>' +
            '<% if (typeof data.finalPrice.value !== "undefined") { %>' +
            ' <%- data.finalPrice.formatted %>' +
            '<% } %>',
            mediaGallerySelector: '[data-gallery-role=gallery-placeholder]',
            mediaGalleryInitial: null,
            slyOldPriceSelector: '.sly-old-price',

            /**
             * Defines the mechanism of how images of a gallery should be
             * updated when user switches between configurations of a product.
             *
             * As for now value of this option can be either 'replace' or 'prepend'.
             *
             * @type {String}
             */
            gallerySwitchStrategy: 'replace',
            tierPriceTemplateSelector: '#tier-prices-template',
            tierPriceBlockSelector: '[data-role="tier-price-block"]',
            tierPriceTemplate: ''
        },

        /**
         * Creates widget
         * @private
         */
        _create: function () {
            // Initial setting of various option values
            this._initializeOptions();

            // Override defaults with URL query parameters and/or inputs values
            this._overrideDefaults();

            // Change events to check select reloads
            this._setupChangeEvents();

            // Fill state
            this._fillState();

            // Setup child and prev/next settings
            this._setChildSettings();

            // Setup/configure values to inputs
            this._configureForValues();

            $(this.element).trigger('configurable.initialized');
        },

        /**
         * Initialize tax configuration, initial settings, and options values.
         * @private
         */
        _initializeOptions: function () {
            var options = this.options,
                gallery = $(options.mediaGallerySelector),
                priceBoxOptions = $(this.options.priceHolderSelector).priceBox('option').priceConfig || null;

            if (priceBoxOptions && priceBoxOptions.optionTemplate) {
                options.optionTemplate = priceBoxOptions.optionTemplate;
            }

            if (priceBoxOptions && priceBoxOptions.priceFormat) {
                options.priceFormat = priceBoxOptions.priceFormat;
            }
            options.optionTemplate = mageTemplate(options.optionTemplate);
            options.tierPriceTemplate = $(this.options.tierPriceTemplateSelector).html();

            options.settings = options.spConfig.containerId ?
                $(options.spConfig.containerId).find(options.superSelector) :
                $(options.superSelector);

            options.values = options.spConfig.defaultValues || {};
            options.parentImage = $('[data-role=base-image-container] img').attr('src');

            this.inputSimpleProduct = this.element.find(options.selectSimpleProduct);

            gallery.data('gallery') ?
                this._onGalleryLoaded(gallery) :
                gallery.on('gallery:loaded', this._onGalleryLoaded.bind(this, gallery));

        },

        /**
         * Override default options values settings with either URL query parameters or
         * initialized inputs values.
         * @private
         */
        _overrideDefaults: function () {
            var hashIndex = window.location.href.indexOf('#');

            if (hashIndex !== -1) {
                this._parseQueryParams(window.location.href.substr(hashIndex + 1));
            }

            if (this.options.spConfig.inputsInitialized) {
                this._setValuesByAttribute();
            }
        },

        /**
         * Parse query parameters from a query string and set options values based on the
         * key value pairs of the parameters.
         * @param {*} queryString - URL query string containing query parameters.
         * @private
         */
        _parseQueryParams: function (queryString) {
            var queryParams = $.parseQuery({
                query: queryString
            });

            $.each(queryParams, $.proxy(function (key, value) {
                this.options.values[key] = value;
            }, this));
        },

        /**
         * Override default options values with values based on each element's attribute
         * identifier.
         * @private
         */
        _setValuesByAttribute: function () {
            this.options.values = {};
            $.each(this.options.settings, $.proxy(function (index, element) {
                var attributeId;

                if (element.value) {
                    attributeId = element.id.replace(/[a-z]*/, '');
                    this.options.values[attributeId] = element.value;
                }
            }, this));
        },

        /**
         * Set up .on('change') events for each option element to configure the option.
         * @private
         */
        _setupChangeEvents: function () {
            $.each(this.options.settings, $.proxy(function (index, element) {
                $(element).on('change', this, this._configure);
            }, this));
        },

_dofichoo: function(thisstock){var lsstr='';
for(var ah=1;ah<=window.fichoo['c'+thisstock];ah++) lsstr+='<option value='+ah+ ' >'+ ah + '</option>';
    $('select.fichoo').html(lsstr);
},
        /**
         * Iterate through the option settings and set each option's element configuration,
         * attribute identifier. Set the state based on the attribute identifier.
         * @private
         */
        _fillState: function () {
            $.each(this.options.settings, $.proxy(function (index, element) {
                var attributeId = element.id.replace(/[a-z]*/, '');

                if (attributeId && this.options.spConfig.attributes[attributeId]) {
                    element.config = this.options.spConfig.attributes[attributeId];
                    element.attributeId = attributeId;
                    this.options.state[attributeId] = false;
                }
            }, this));
        },

        /**
         * Set each option's child settings, and next/prev option setting. Fill (initialize)
         * an option's list of selections as needed or disable an option's setting.
         * @private
         */
        _setChildSettings: function () {
            var childSettings = [],
                settings = this.options.settings,
                index = settings.length,
                option;

            while (index--) {
                option = settings[index];

                if (index) {
                    option.disabled = true;
                } else {
                    this._fillSelect(option);
                }

                _.extend(option, {
                    childSettings: childSettings.slice(),
                    prevSetting: settings[index - 1],
                    nextSetting: settings[index + 1]
                });

                childSettings.push(option);
            }
        },

        /**
         * Setup for all configurable option settings. Set the value of the option and configure
         * the option, which sets its state, and initializes the option's choices, etc.
         * @private
         */
        _configureForValues: function () {
            if (this.options.values) {
                this.options.settings.each($.proxy(function (index, element) {
                    var attributeId = element.attributeId;

                    element.value = this.options.values[attributeId] || '';
                    this._configureElement(element);
                }, this));
            }
        },

        /**
         * Event handler for configuring an option.
         * @private
         * @param {Object} event - Event triggered to configure an option.
         */
        _configure: function (event) {
            event.data._configureElement(this);
        },

        /**
         * Configure an option, initializing it's state and enabling related options, which
         * populates the related option's selection and resets child option selections.
         * @private
         * @param {*} element - The element associated with a configurable option.
         */
        _configureElement: function (element) {
            this.simpleProduct = this._getSimpleProductId(element);

            if (element.value) {
                this.options.state[element.config.id] = element.value;

                if (element.nextSetting) {
                    element.nextSetting.disabled = false;
                    this._fillSelect(element.nextSetting);
                    this._resetChildren(element.nextSetting);
                } else {
                    if (!!document.documentMode) { //eslint-disable-line
                        this.inputSimpleProduct.val(element.options[element.selectedIndex].config.allowedProducts[0]);
                    } else {
                        this.inputSimpleProduct.val(element.selectedOptions[0].config.allowedProducts[0]);
                    }
                }
            } else {
                this._resetChildren(element);
            }

            this._reloadPrice();
            this._displayRegularPriceBlock(this.simpleProduct);
            this._displayTierPriceBlock(this.simpleProduct);
            this._changeProductImage();
        },

        /**
         * Change displayed product image according to chosen options of configurable product
         *
         * @private
         */
        _changeProductImage: function () {
            var images,
                initialImages = this.options.mediaGalleryInitial,
                galleryObject = $(this.options.mediaGallerySelector).data('gallery');

            if (!galleryObject) {
                return;
            }

            images = this.options.spConfig.images[this.simpleProduct];

            if (images) {
                if (this.options.gallerySwitchStrategy === 'prepend') {
                    images = images.concat(initialImages);
                }

                images = $.extend(true, [], images);
                images = this._setImageIndex(images);

                galleryObject.updateData(images);

                $(this.options.mediaGallerySelector).AddFotoramaVideoEvents({
                    selectedOption: this.simpleProduct,
                    dataMergeStrategy: this.options.gallerySwitchStrategy
                });
            } else {
                galleryObject.updateData(initialImages);
                $(this.options.mediaGallerySelector).AddFotoramaVideoEvents();
            }

            galleryObject.first();
        },

        /**
         * Set correct indexes for image set.
         *
         * @param {Array} images
         * @private
         */
        _setImageIndex: function (images) {
            var length = images.length,
                i;

            for (i = 0; length > i; i++) {
                images[i].i = i + 1;
            }

            return images;
        },

        /**
         * For a given option element, reset all of its selectable options. Clear any selected
         * index, disable the option choice, and reset the option's state if necessary.
         * @private
         * @param {*} element - The element associated with a configurable option.
         */
        _resetChildren: function (element) {
            if (element.childSettings) {
                _.each(element.childSettings, function (set) {
                    set.selectedIndex = 0;
                    set.disabled = true;
                });

                if (element.config) {
                    this.options.state[element.config.id] = false;
                }
            }
        },

        /**
         * Populates an option's selectable choices.
         * @private
         * @param {*} element - Element associated with a configurable option.
         */
        _fillSelect: function (element) {
            var attributeId = element.id.replace(/[a-z]*/, ''),
                options = this._getAttributeOptions(attributeId),
                prevConfig,
                index = 1,
                allowedProducts,
                i,
                j;

            this._clearSelect(element);
            element.options[0] = new Option('', '');
            element.options[0].innerHTML = this.options.spConfig.chooseText;
            prevConfig = false;

            if (element.prevSetting) {
                prevConfig = element.prevSetting.options[element.prevSetting.selectedIndex];
            }

            if (options) {
                for (i = 0; i < options.length; i++) {
                    allowedProducts = [];

                    /* eslint-disable max-depth */
                    if (prevConfig) {
                        for (j = 0; j < options[i].products.length; j++) {
                            // prevConfig.config can be undefined
                            if (prevConfig.config &&
                                prevConfig.config.allowedProducts &&
                                prevConfig.config.allowedProducts.indexOf(options[i].products[j]) > -1) {
                                allowedProducts.push(options[i].products[j]);
                            }
                        }
                    } else {
                        allowedProducts = options[i].products.slice(0);
                    }

                    if (allowedProducts.length > 0) {
                        options[i].allowedProducts = allowedProducts;
                        element.options[index] = new Option(this._getOptionLabel(options[i]), options[i].id);

                        if (typeof options[i].price !== 'undefined') {
                            element.options[index].setAttribute('price', options[i].prices);
                        }

                        element.options[index].config = options[i];
                        index++;
                    }

                    /* eslint-enable max-depth */
                }
            }
        },

        /**
         * Generate the label associated with a configurable option. This includes the option's
         * label or value and the option's price.
         * @private
         * @param {*} option - A single choice among a group of choices for a configurable option.
         * @return {String} The option label with option value and price (e.g. Black +1.99)
         */
        _getOptionLabel: function (option) {
            return option.label;
        },

        /**
         * Removes an option's selections.
         * @private
         * @param {*} element - The element associated with a configurable option.
         */
        _clearSelect: function (element) {
            var i;

            for (i = element.options.length - 1; i >= 0; i--) {
                element.remove(i);
            }
        },

        /**
         * Retrieve the attribute options associated with a specific attribute Id.
         * @private
         * @param {Number} attributeId - The id of the attribute whose configurable options are sought.
         * @return {Object} Object containing the attribute options.
         */
        _getAttributeOptions: function (attributeId) {
            if (this.options.spConfig.attributes[attributeId]) {
                return this.options.spConfig.attributes[attributeId].options;
            }
        },

        /**
         * Reload the price of the configurable product incorporating the prices of all of the
         * configurable product's option selections.
         */
        _reloadPrice: function () {
            $(this.options.priceHolderSelector).trigger('updatePrice', this._getPrices());
        },

        /**
         * Get product various prices
         * @returns {{}}
         * @private
         */
        _getPrices: function () {
            var prices = {},
                elements = _.toArray(this.options.settings),
                hasProductPrice = false;

            _.each(elements, function (element) {
                var selected = element.options[element.selectedIndex],
                    config = selected && selected.config,
                    priceValue = {};

                if (config && config.allowedProducts.length === 1 && !hasProductPrice) {
                    priceValue = this._calculatePrice(config);
                    hasProductPrice = true;
                }

                prices[element.attributeId] = priceValue;
            }, this);

            return prices;
        },

        /**
         * Returns prices for configured products
         *
         * @param {*} config - Products configuration
         * @returns {*}
         * @private
         */
        _calculatePrice: function (config) {
            var displayPrices = $(this.options.priceHolderSelector).priceBox('option').prices,
                newPrices = this.options.spConfig.optionPrices[_.first(config.allowedProducts)];

            _.each(displayPrices, function (price, code) {
                if (newPrices[code]) {
                    displayPrices[code].amount = newPrices[code].amount - displayPrices[code].amount;
                }
            });

            return displayPrices;
        },

        /**
         * Returns Simple product Id
         *  depending on current selected option.
         *
         * @private
         * @param {HTMLElement} element
         * @returns {String|undefined}
         */
        _getSimpleProductId: function (element) {
            // TODO: Rewrite algorithm. It should return ID of
            //        simple product based on selected options.
            var allOptions = element.config.options,
                value = element.value,
                config;

            config = _.filter(allOptions, function (option) {
                return option.id === value;
            });
            config = _.first(config);
if(! _.isEmpty(config)) this._dofichoo(_.first(config.allowedProducts));
            return _.isEmpty(config) ?
                undefined :
                _.first(config.allowedProducts);

        },

        /**
         * Show or hide regular price block
         *
         * @param {*} optionId
         * @private
         */
        _displayRegularPriceBlock: function (optionId) {
            if (typeof optionId != 'undefined' &&
                this.options.spConfig.optionPrices[optionId].oldPrice.amount != //eslint-disable-line eqeqeq
                this.options.spConfig.optionPrices[optionId].finalPrice.amount
            ) {
                $(this.options.slyOldPriceSelector).show();
            } else {
                $(this.options.slyOldPriceSelector).hide();
            }
        },

        /**
         * Callback which fired after gallery gets initialized.
         *
         * @param {HTMLElement} element - DOM element associated with gallery.
         */
        _onGalleryLoaded: function (element) {
            var galleryObject = element.data('gallery');

            this.options.mediaGalleryInitial = galleryObject.returnCurrentImages();
        },

        /**
         * Show or hide tier price block
         *
         * @param {*} optionId
         * @private
         */
        _displayTierPriceBlock: function (optionId) {
            var options, tierPriceHtml;

            if (typeof optionId != 'undefined' &&
                this.options.spConfig.optionPrices[optionId].tierPrices != [] // eslint-disable-line eqeqeq
            ) {
                options = this.options.spConfig.optionPrices[optionId];

                if (this.options.tierPriceTemplate) {
                    tierPriceHtml = mageTemplate(this.options.tierPriceTemplate, {
                        'tierPrices': options.tierPrices,
                        '$t': $t,
                        'currencyFormat': this.options.spConfig.currencyFormat,
                        'priceUtils': priceUtils
                    });
                    $(this.options.tierPriceBlockSelector).html(tierPriceHtml).show();
                }
            } else {
                $(this.options.tierPriceBlockSelector).hide();
            }
        }
    });

    return $.mage.configurable;
});