Main.js

function getFileExtension(fullFileName){
    const extension = fullFileName.split('.').pop();
    return extension;
}

function getFileName(fullFileName){
    var fileName = fullFileName.substr(0, fullFileName.lastIndexOf('.'));
    return fileName;
}

function isImageFile(fullFileName){
    return (/\.(gif|jpe?g|tiff?|png|webp|bmp)$/i).test(fullFileName);
}

function checkAnImageExists(dataSet,fileNameWithoutExtension, startIndex){
    let imageIndex = -1;
    
    for(let j=startIndex; j<dataSet.length; j++){
        const currentFileName = getFileName(dataSet[j]);
        if(isImageFile(dataSet[j]) &&  currentFileName == fileNameWithoutExtension){
            imageIndex = j;
            break;
        }
    }
    
    return imageIndex;
}

let dataSet = ["a.md", "a.jpg", "b.md", "b.png"];
//let dataSet = ["a.md", "a.jpg", "b.md", "b.png", "c.md"];

let newData = [];

for(let i=0; i<dataSet.length; i++){
    console.log("Loop: ", i);
    if(["md", "MD"].indexOf(getFileExtension(dataSet[i])) > -1 ){
        
        console.log("Content: ", dataSet[i]);
        let content = dataSet[i];
        let image = "";
        
        let imageIndex = checkAnImageExists(dataSet, getFileName(dataSet[i]), i);
        
        if(imageIndex != -1){
            image = dataSet[imageIndex];
            newData.push({content, image });
        }else{
            console.log(`Image file is missing for ${dataSet[i]}`);
            newData = [];
            break;
        }
    }else{
        continue;
    }
}

for(let i=0; i<newData.length; i++){
    console.log(newData[i]);
}