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<h1 class="page-title">ensureAsync.js</h1>
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<pre class="prettyprint source linenums"><code>import setImmediate from './internal/setImmediate';
import { isAsync } from './internal/wrapAsync';
/**
* Wrap an async function and ensure it calls its callback on a later tick of
* the event loop. If the function already calls its callback on a next tick,
* no extra deferral is added. This is useful for preventing stack overflows
* (`RangeError: Maximum call stack size exceeded`) and generally keeping
* [Zalgo](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony)
* contained. ES2017 `async` functions are returned as-is -- they are immune
* to Zalgo's corrupting influences, as they always resolve on a later tick.
*
* @name ensureAsync
* @static
* @memberOf module:Utils
* @method
* @category Util
* @param {AsyncFunction} fn - an async function, one that expects a node-style
* callback as its last argument.
* @returns {AsyncFunction} Returns a wrapped function with the exact same call
* signature as the function passed in.
* @example
*
* function sometimesAsync(arg, callback) {
* if (cache[arg]) {
* return callback(null, cache[arg]); // this would be synchronous!!
* } else {
* doSomeIO(arg, callback); // this IO would be asynchronous
* }
* }
*
* // this has a risk of stack overflows if many results are cached in a row
* async.mapSeries(args, sometimesAsync, done);
*
* // this will defer sometimesAsync's callback if necessary,
* // preventing stack overflows
* async.mapSeries(args, async.ensureAsync(sometimesAsync), done);
*/
export default function ensureAsync(fn) {
if (isAsync(fn)) return fn;
return function (...args/*, callback*/) {
var callback = args.pop()
var sync = true;
args.push((...innerArgs) => {
if (sync) {
setImmediate(() => callback(...innerArgs));
} else {
callback(...innerArgs);
}
});
fn.apply(this, args);
sync = false;
};
}
</code></pre>
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