Updating value of variable in Node js promise function

Question

I am trying to update value of a variable inside the promise function. What am I doing wrong?

var data="valueoutside";

wb.xlsx.readFile(filePath).then(function(){

data="new value";
}
);

console.log(data); 
//shows 'valueoutside' as output and not 'new value'

Answer 1

readFile is asynchronous, therefore console.log(data) is called before readFile has a chance to finish running and updating the value.

Change your code to this and it will work:

var data="valueoutside";

wb.xlsx.readFile(filePath).then(function() {
    data="new value";
    console.log(data); 
});

Another option would be to use a synchronous call:

var data="valueoutside";
var contents = wb.xlsx.readFileSync(filePath);

data="new value";

console.log(data); 

Answer 2

Move your console.log(data) into the then handler, and you'll see the updated value.

var data="valueoutside";

wb.xlsx.readFile(filePath).then(function(){
    data="new value";
    console.log(data);
});

The function inside the then handler will execute once the promise resolves.

Answer 3

ReadFile is asynchronous function. So you should write console.log in then function

var data="valueoutside";

wb.xlsx.readFile(filePath).then(function(){
   data="new value";
   console.log(data);
});