node.js make code wait until the fs.readFile is completed
Solution 1
If you need to do it synchronously then you should use fs.readFileSync() (https://nodejs.org/api/fs.html#fs_fs_readfilesync_filename_options) instead.
var fs = require('fs');
function myfun(filePath){
return fs.readFileSync(filePath);
}
Solution 2
You need to pass a callback to myfun
function as below in order to get back data from the function when file reading is over:
var fs = require('fs');
function myfun(filePath, cb){
var str = '';
fs.readFile(filePath, 'utf8', function(err, data){
if(err) throw err;
cb(data);
});
}
// call it like this
myfun('some_path', function(data) { /* use returned data here */} );
You need to invest some time into better understanding of asynchronous nature of JavaScript.
The problem with your code is that return str
is outside of the readFile
callback, which means return str
executes earlier than the readFile
callback gets called to set str
to a meaningful value.
Deryckxie
Updated on February 11, 2020Comments
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Deryckxie over 4 years
I got a problem in node.js file system. here is my code. my function always return a empty string. I'm wondering is there anyway to make my function stop execute until readFile method is completed.
var fs = require('fs'); function myfun(filePath){ var str = ''; fs.readFile(filePath, function(err, data){ if(err) throw err; str = data; }); return str; //here, the variable str always return '' because the function doesn't wait for the readFile method complete. }
add explanation
Actually I'm doing something like this: the function myfun is used for replace str you can see my code:
function fillContent(content) { var rex = /\<include.*?filename\s*=\s*"(.+?)"\/>/g; var replaced = fileStr.replace(rex, function (match, p1) { var filePath = p1 var fileContent = ''; fs.readFile(filePath, function (err, data) { if (err) { throw err; } fileContent = data; }); return fileContent; }); return replaced;// here, the return value is used for replacement }
I need a return value in the replace function, so this is why I didn't use a callback function
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Pointy almost 9 years@GopinathShiva puting
return str;
inside the callback will not do any good. OP you're looking forfs.readFileSync()
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Gopinath Shiva almost 9 yearsfor your reference stackoverflow.com/questions/10058814/get-data-from-fs-readfile
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robertklep almost 9 yearsIf you really need to use it with
.replace()
, you should usefs.readFileSync()
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Deryckxie almost 9 yearsthank robertklep, I'm wondering whether I used fs in a replace function is a good way
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adelphus almost 9 yearsThis is the way to go with nodejs - although I would probably add the utf8 encoding parameter because it looks like the OP is expecting a string to be returned.