How to prevent execute below code lines before a const in Javascript

Question

I have a JavaScript code like this

     const toDataURL =  url => fetch(url)
            .then(response => response.blob())
            .then(blob => new Promise((resolve, reject) => {
                const reader = new FileReader()
                 reader.onloadend = () => resolve(reader.result)
                reader.onerror = reject
                reader.readAsDataURL(blob)
            }))
         toDataURL(hcmImage)
            .then(dataUrl => {
                console.log('RESULT:', dataUrl)
                hcmImageString = dataUrl.substring(dataUrl.indexOf('base64,') + 7)
                alert(dataUrl);
                alert(hcmImageString);
            })

This is implemented by another person. I want to add async to this. because sometimes code executes below code lines before executing this part.

I tried to add an async keyword like this. But it is not working

           const toDataURL = async url => fetch(url)
            .then(response => response.blob())
            .then(blob => new Promise((resolve, reject) => {
                const reader = new FileReader()
                 reader.onloadend = () => resolve(reader.result)
                reader.onerror = reject
                reader.readAsDataURL(blob)
            }))
         toDataURL(hcmImage)
            .then(dataUrl => {
                console.log('RESULT:', dataUrl)
                hcmImageString = dataUrl.substring(dataUrl.indexOf('base64,') + 7)
                alert(dataUrl);
                alert(hcmImageString);
            })

Please give me a solution to prevent executing blow code lines before this. Thank you