在setTimeout中调用一个函数

Question

Defining a function and calling "callback"

var evenDoubler = function(number, callback) {
    if (number % 2 == 0) {
        setTimeout(function() {
             callback(null, number * 2)
        }, 100);
    }
}

Directly calling "callback"

var evenDoubler = function(number, callback) {
    if (number % 2 == 0) {
        setTimeout(callback(null, number * 2), 100);
    }
}

Whats the difference between these 2 approaches?

Answer 1

When you "directly" call callback() , that happens before the timer goes off; it happens before the call to setTimeout() happens.

An expression like

callback(null, number * 2)

is a function call, and as such it's always evaluated when JavaScript needs its value. When it's used in a function call:

setTimeout(callback(null, number * 2), 100);

then it's evaluated in order to get the value that needs to be passed to setTimeout() . It's the same as if you call some function with a simple arithmetic expression:

someFunction(x + y);

You fully expect that what's passed to the function is the sum of x and y . A function call is just another sort of expression, and it too will be fully evaluated when it's used as an argument to a function.

Answer 2

setTimeout takes as first argument a function reference. So when you call a function (as in your second case), that function call should return a function. This is not your case. You want to pass the function reference -- so don't call it. You could do it with bind which does return a new function, with the arguments already bound to it:

var evenDoubler = function(number, callback) {
    if (number % 2 == 0) {
        setTimeout(callback.bind(null, null, number * 2), 100);
    }
}

Answer 3

Question is already answered but I want to chime in as I have tested this solution when I was going through the same course.

If you really want to do the "dirty" call in a neat way then give the first argument as just the function name, rest of the parameters after timeout period, as explained here:

https://nodejs.org/en/docs/guides/timers-in-node/

going by above explanation I tested the code with below and worked:

var eventDoubler = function (num, callback) {
    var waitTime = Math.floor(Math.random()*(maxTime+1));

    if( num % 2 ) {
        setTimeout(callback, waitTime, new Error('Odd Input'));
    } else {
        setTimeout(callback, waitTime, null, num*2 , waitTime );
    }
};