setTimeout - callback argument must be a function

Question

My code was working until i updated node.js to version 8.11.3

Now i always get error "callback argument must be a function" when trying to call a function with setTimeout.

function testFunction(itemid, price) {

  var url = 'https://example.com';
  var options = {
  method: 'get',
  url: url
  }

  request(options, function (err, res, body) {
    var response = JSON.parse(body);

     if(response.status == 'fail'){
        setTimeout(testFunction(itemid, price), 100);
     }
  })

}

Solution 1

Callback argument for setTimeout must be a function. Write it like this. Not tested but it should work.

function testFunction(itemid, price) {

    var url = 'https://example.com';
    var options = {
        method: 'get',
        url: url
    }

    request(options, function (err, res, body) {
        var response = JSON.parse(body);
        if(response.status == 'fail'){
            setTimeout(function () {
                testFunction(itemid, price);
            }, 100);
        }
    })
}

Solution 2

@keyur is correct. According to the node.js timers documentation the setTimeout function takes the name of the function to execute as the first argument, the delay in milliseconds as the second argument, followed by any arguments being passed to the function.

For Example:

setTimeout(testFunction, 100, itemid, price);

Solution 3

Yes, setTimeout() expects first argument as callback function. we can make use of es6 fat arrow function here you can try this!

setTimeout(() => testFunction(itemid, price), 100);

Hope this helps!

Comments

[Estus Flask]

The code never worked as intended, FYI.

[Patrick Hund]

Agreed, @MHH, you probably just didn't notice that testFunction was not called after 100 ms, but rather immediately – you weren't passing the function testFunction as argument to setTimeout, but rather the result from calling it. Upgrading to Node 8.11.3 exposed your mistake, because it doesn't allow you to pass anything but a function anymore

[Estus Flask]

it always requires a function as the callback argument - it doesn't. setTimeout(<string>) is a form of eval that was intentionally removed from Node implementation.

[Keyur Ramoliya]

@estus Don't about that. Revoking that line.