const Category = require('../../models/Category');


const fieldCheck = (req, res, next) => {
    const data = req.body;
    const errors = [];
    for( i = 0; i < data.length ; i++){
    Category.findOne({ category_name : data[i]})
    .then(user => {
        if(user){
            // # If a reqistered User ID is found ,then move ahead
            errors[i] = 'Duplicate Entry Found';
            errors.push(errors[i]);
        } 
    }).catch(err =>{
        return res.json(err);
        }
    )
    }
    console.log(errors);
};

module.exports = fieldCheck;

回答如下：

您正在尝试调用一个同步环（findOne）内的异步方法（for）。当你的经验，这就像油和水。

一个简单的办法就是让你的方法异步和使用await关键字，例如：

const fieldCheck = async (req, res, next) => {
    const data = req.body;
    const errors = [];
    try {
        for( i = 0; i < data.length ; i++) {
            let user = await Category.findOne({ category_name : data[i]});
            if (user) {
                // # If a reqistered User ID is found ,then move ahead
                errors[i] = 'Duplicate Entry Found';
                errors.push(errors[i]);
            }
        }
        // I assume you wanted to respond to res.json here?
        console.log(errors);
    } catch (err) {
        return res.json(err);
    }
};

module.exports = fieldCheck;