node.js then() not working - node.js

Question

I am new to node.js so I am trying to understand promises and wait on node.js. I want to print the file note.txt.
Here is my code
var fs = require('fs');
fs.readFile('note.txt','utf8').then(contents => console.log(contents))
.catch(err => console.error(err));
When I run above code. I get the following error.
fs.readFile('note.txt','utf8').then(contents => console.log(contents))
TypeError: Cannot read property 'then' of undefined
at Object.<anonymous> (/Applications/nodeApps/test/index.js:13:31)
at Module._compile (module.js:635:30)
at Object.Module._extensions..js (module.js:646:10)
at Module.load (module.js:554:32)
at tryModuleLoad (module.js:497:12)
at Function.Module._load (module.js:489:3)
at Function.Module.runMain (module.js:676:10)
at startup (bootstrap_node.js:187:16)
at bootstrap_node.js:608:3
And I try another method for the same thing.
var fs = require('fs');
async function read_file(){
var file_data = await fs.readFile('note.txt','utf8');
return file_data;
}
console.log(read_file());
And I get following error
Promise { <pending> }
(node:6532) [DEP0013] DeprecationWarning: Calling an asynchronous function without callback is deprecated.
I get the same error when I run with --harmony. I m not sure if there is bug on my code or what is wrong. Please help me understand.
My Environment
Node version: v8.9.0
node -p process.versions.v8: 6.1.534.46

Answer 1

You're getting errors because fs.readfile doesn't return a promise; hence then doesn't exist. For you to use the function as a promise, you will need to wrap it up as a promise; you could use something like bluebird or Q.

Answer 2

Thank you for the answers. I learned that function must return promise in order to use then() and catch(). So the code should be like this
var fs = require('fs');
function read_file(){
return new Promise(function(resolve, reject) {
fs.readFile('note.txt','utf8',function(err,file){
if(err){
return reject(err);
}else{
resolve(file);
}
});
});
}
read_file().then(
(data)=>{
console.log('success: '+data);
}
).catch((err)=>{
console.log('error: ',err);
});

Answer 3

If you use NodeJs v10, try fs.promises:
var fs = require('fs').promises; // v10.0 use require('fs/promises')
fs.readFile('note.txt','utf8').then(contents => console.log(contents))
.catch(err => console.error(err));
If not, use readFileSync:
// This code can use for node v10 or lowwer
var fs = require('fs');
var data = fs.readFileSync('a.json');
console.log(data);

Answer 4

try to use the async await
function (async err => {
if (err) {
console.err ....}
await .... <other function included or comes after then .>
await ... <other function included>
})