Not getting the expected data from file awlays - node.js

Question

Using node.js, I have created a 'search(string)' function which sends API request, captures the response payload object and write it to a file.
In my test file when I call that function in two different tests with different parameters and reading the saved response. My assertion is failing sometimes and I could see that data comparison from the file is mixing up from the first function calling with 2nd function calling data or vice versa.
Looks like sometimes reading file is happening before writing to the file. Any thoughts?
Code example:
client.js:
const search = function(t)
{
fetch(`http://www.example.com/?t=${t}&apikey=${apiKey}`)
.then(function (response) {
return response.json();
})
.then( ( function(data){
writeData(data)
}))
.catch(function (err) {
console.log('error: ' + err);
});
function writeData(data)
{
let result = JSON.stringify(data);
fs.writeFileSync('./filepath/resultData.json', result)
}
--------------------------------------------------------------------------
test.js
function 1()
{
search('A')
rawData = fs.readFileSync('./filepath/resultData.json')
r = JSON.parse(rawData);
console.log(r.Title)
}
function 2()
{
search('B')
rawData = fs.readFileSync('./filepath/resultData.json')
r = JSON.parse(rawData);
console.log(r.Title)
}
sometimes I get the same value for r.title and sometimes different.

Answer 1

Your search function has an async fetch call call, which resolves way after your test runs... Have your function return the promise and use the .then to check for validity of data.
const search = function(t) {
return fetch(`http://www.example.com/?t=${t}&apikey=${apiKey}`)
.then(function(response) {
return response.json();
})
.then((function(data) {
writeData(data);
return data; // not really required in your case, but to make it work in a more "promise like" fashion.
}))
.catch(function(err) {
console.log('error: ' + err);
return error;
});
}
function writeData(data) {
let result = JSON.stringify(data);
fs.writeFileSync('./filepath/resultData.json', result);
}
--------------------------------------------------------------------------
test.js
function 1() {
search('A').then(() => {
rawData = fs.readFileSync('./filepath/resultData.json')
r = JSON.parse(rawData);
console.log(r.Title);
});
}
function 2() {
search('B').then(() => {
rawData = fs.readFileSync('./filepath/resultData.json')
r = JSON.parse(rawData);
console.log(r.Title)
});
}

Answer 2

The search method calls fetchwhich is executed asynchrously. This means that the code reading the data might be executed before the result of the fetch has been written to the file.
This is a race condition, it is not predictable which happens first, writing or reading the data.
function 1()
{
search('A') // Fetch data is executed, data gets written in `then` block
// which means that the data gets written once fetching the
// data has been completed.
// The time this operation needs depends on the network
// speed and server load for example.
// The code below might get executed before the fetch has
// finished and the data has been written to disk
rawData = fs.readFileSync('./filepath/resultData.json')
r = JSON.parse(rawData);
console.log(r.Title)
}
One way to solve this is to re-structure the flow so that function 1 and function 2 are passed as callback to the search function as follows (in fact, it is not necessary to have function 2 in this case). Additionally writing/reading to/from disk can be omitted as well by passing the data directly to function1 when calling it as callback inside the search function:
const search = function(t, callback)
{
fetch(`http://www.example.com/?t=${t}&apikey=${apiKey}`)
.then(function (response) {
return response.json();
})
.then( ( function(data){
callback(data)
}))
.catch(function (err) {
console.log('error: ' + err);
});
--------------------------------------------------------------------------
test.js
function function1(data)
{
r = JSON.parse(rawData);
console.log(r.Title)
}
search('A', function1)
search('B', function1)