=> Go to your theme Folder and create file called addtocart.phtml
and
past below code.
Quantity Increment and Decrement Button working fine by using below code
E.g :
app/design/frontend/Vendor/theme/Magento_Catalog/templates/product/view/addtocart.phtml
<?php
/**
* Copyright © 2013-2017 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()): ?>
<div class="field qty">
<label class="label" for="qty"><span><?php /* @escapeNotVerified */ echo __('Qty') ?></span></label>
<?php /*Add Plus Button */ ?>
<input type='button' value='+' class='qtyplus' field='qty' />
<div class="control">
<input type="number"
name="qty"
id="qty"
maxlength="12"
value="<?php /* @escapeNotVerified */ echo $block->getProductDefaultQty() * 1 ?>"
title="<?php /* @escapeNotVerified */ echo __('Qty') ?>" class="input-text qty"
data-validate="<?php echo $block->escapeHtml(json_encode($block->getQuantityValidators())) ?>"
/>
<?php /*Add Minus Button */ ?>
<input type='button' value='-' class='qtyminus' field='qty' />
</div>
</div>
<?php endif; ?>
<div class="actions">
<button type="submit"
title="<?php /* @escapeNotVerified */ echo $buttonTitle ?>"
class="action primary tocart"
id="product-addtocart-button">
<span><?php /* @escapeNotVerified */ echo $buttonTitle ?></span>
</button>
<?php echo $block->getChildHtml('', true) ?>
</div>
</div>
</div>
<?php endif; ?>
<script type="text/x-magento-init">
{
"#product_addtocart_form": {
"Magento_Catalog/product/view/validation": {
"radioCheckboxClosest": ".nested"
}
}
}
</script>
<?php if (!$block->isRedirectToCartEnabled()) : ?>
<script type="text/x-magento-init">
{
"#product_addtocart_form": {
"catalogAddToCart": {
"bindSubmit": false
}
}
}
</script>
<?php endif; ?>
<?php /*JS FOR +-*/ ?>
<script type="text/javascript">
//<![CDATA[
require(['jquery'], function ($) {
setTimeout(function () {
// This button will increment the value
jQuery('.qtyplus').click(function (e) {
// Stop acting like a button
e.preventDefault();
// Get the field name
fieldName = jQuery(this).attr('field');
// Get its current value
var currentVal = parseInt(jQuery('input[name=' + fieldName + ']').val());
// If is not undefined
if (!isNaN(currentVal)) {
// Increment
jQuery('input[name=' + fieldName + ']').val(currentVal + 1);
} else {
// Otherwise put a 0 there
jQuery('input[name=' + fieldName + ']').val(0);
}
});
// This button will decrement the value till 0
jQuery(".qtyminus").click(function (e) {
// Stop acting like a button
e.preventDefault();
// Get the field name
fieldName = jQuery(this).attr('field');
// Get its current value
var currentVal = parseInt(jQuery('input[name=' + fieldName + ']').val());
// If it isn't undefined or its greater than 0
if (!isNaN(currentVal) && currentVal > 0) {
// Decrement one
jQuery('input[name=' + fieldName + ']').val(currentVal - 1);
} else {
// Otherwise put a 0 there
jQuery('input[name=' + fieldName + ']').val(0);
}
});
}, 100);
});
//]]>
</script>