javascript : Async/await in .replace

Question

I am using the async/await function the following way

async function(){   let output = await string.replace(regex, async (match)=>{     let data = await someFunction(match)     console.log(data); //gives correct data     return data   })   return output; } 

But the returned data is an promise object. Just confused about the way it should be implemented in such functions with callback.

Answer 1

An easy function to use and understand for some async replace :

async function replaceAsync(str, regex, asyncFn) {     const promises = [];     str.replace(regex, (match, ...args) => {         const promise = asyncFn(match, ...args);         promises.push(promise);     });     const data = await Promise.all(promises);     return str.replace(regex, () => data.shift()); } 

It does the replace function twice so watch out if you do something heavy to process. For most usages though, it's pretty handy.

Use it like this:

replaceAsync(myString, /someregex/g, myAsyncFn)     .then(replacedString => console.log(replacedString)) 

Or this:

const replacedString = await replaceAsync(myString, /someregex/g, myAsyncFn); 

Don't forget that your myAsyncFn has to return a promise.

An example of asyncFunction :

async function myAsyncFn(match) {     // match is an url for example.     const fetchedJson = await fetch(match).then(r => r.json());     return fetchedJson['date']; }  function myAsyncFn(match) {     // match is a file     return new Promise((resolve, reject) => {         fs.readFile(match, (err, data) => {             if (err) return reject(err);             resolve(data.toString())         });     }); } 

Answer 2

The native replace method does not deal with asynchronous callbacks, you cannot use it with a replacer that returns a promise.

We can however write our own replace function that deals with promises:

async function(){   return string.replace(regex, async (match)=>{     let data = await someFunction(match)     console.log(data); //gives correct data     return data;   }) }  function replaceAsync(str, re, callback) {     // http://es5.github.io/#x15.5.4.11     str = String(str);     var parts = [],         i = 0;     if (Object.prototype.toString.call(re) == "[object RegExp]") {         if (re.global)             re.lastIndex = i;         var m;         while (m = re.exec(str)) {             var args = m.concat([m.index, m.input]);             parts.push(str.slice(i, m.index), callback.apply(null, args));             i = re.lastIndex;             if (!re.global)                 break; // for non-global regexes only take the first match             if (m[0].length == 0)                 re.lastIndex++;         }     } else {         re = String(re);         i = str.indexOf(re);         parts.push(str.slice(0, i), callback.apply(null, [re, i, str]));         i += re.length;     }     parts.push(str.slice(i));     return Promise.all(parts).then(function(strings) {         return strings.join("");     }); }