The following function takes and image from an url, loads it, and returns its width and height:
function getImageData (url) {
const img = new Image()
img.addEventListener('load', function () {
return { width: this.naturalWidth, height: this.naturalHeight }
})
img.src = url
}
The problem is, if I do something like this:
ready () {
console.log(getImageData(this.url))
}
I get undefined
because the function runs but the imaged hasn't loaded yet.
How to use await/async to return the value only when the photo has loaded and the width and height is already available?
If the callback function returns non-error output, we resolve the Promise with the output. Let's start by converting a callback to a promise for a function that accepts a fixed number of parameters: const fs = require('fs'); const readFile = (fileName, encoding) => { return new Promise((resolve, reject) => { fs.
Async/Await is used to work with promises in asynchronous functions. It is basically syntactic sugar for promises. It is just a wrapper to restyle code and make promises easier to read and use. It makes asynchronous code look more like synchronous/procedural code, which is easier to understand.
async and await Inside an async function, you can use the await keyword before a call to a function that returns a promise. This makes the code wait at that point until the promise is settled, at which point the fulfilled value of the promise is treated as a return value, or the rejected value is thrown.
The function that takes another function as an argument is called a higher-order function. According to this definition, any function can become a callback function if it is passed as an argument. Callbacks are not asynchronous by nature, but can be used for asynchronous purposes.
How to use
async
/await
to turn this callback function into a promise?
You don't. As usual, you use the new Promise
constructor. There's no syntactic sugar for that.
function loadImage(url) {
return new Promise((resolve, reject) => {
const img = new Image();
img.addEventListener('load', () => resolve(img));
img.addEventListener('error', reject); // don't forget this one
img.src = url;
});
}
How to use
await
/async
to log the value only when the photo has loaded and the width and height is already available?
You can do
async function getImageData(url) {
const img = await loadImage(url);
return { width: img.naturalWidth, height: img.naturalHeight };
}
async function ready() {
console.log(await getImageData(this.url))
}
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