将 JS 承诺值分配给变量

Question

As a starter in JS promises it is a bit confusing to me, I am trying to get a base64 of an image using this function:

const convertFileToBase64 = file => new Promise((resolve, reject) => {
 const reader = new FileReader();
 reader.readAsDataURL(file);
 reader.onload = () => resolve(reader.result);
 reader.onerror = reject;
})

when I call this function I get a promise I can use.then to print the value, but I cannot assign the returned value of the function to a variable.

Answer 1

When calling a function that returns a Promise there are 2 ways to get the result from it:

1. using .then
2. using async/await

.then approach:

convertFileToBase64(myFile)
  .then(result => {
    // do something with the result within this little function
  })
  .catch(err => {
    // handle any error
  });

async/await approach:

// must be inside an async function
async function doSomething() {
  const result = await convertFileToBase64(myFile);  // wait and assign result to variable
  // do something with the result
  // errors can get handled by whoever calls this async function
}