如果参数为“未定义”，为什么“clearTimeout”不会产生错误？

Question

 function getData() { console.log('Fetching data'); } const magic = (fn, delay) => { let timer; return function() { clearTimeout(timer); timer = setTimeout(() => { fn(); }, delay); } } const better = magic(getData, 3000);

 <input type="text" onkeypress="better()" />

For the very first time timer would be undefined then why JavaScript doesn't give an error for that?

Answer 1

If you pass an handleid that is not present in the list of active timeouts, then the method ignores it. It doesn't throw an error.

The clearTimeout() method must clear the entry identified as handle from the list of active timeouts of the WindowTimers object on which the method was invoked, where handle is the argument passed to the method, if any. (If handle does not identify an entry in the list of active timeouts of the WindowTimers object on which the method was invoked, the method does nothing .)

Reference

Answer 2

That's because clearTimeout also handles undefined, meaning it has something like a check within the implementation of the function eg

const clearTimeout = (timer) => {
    if (timer) {
        // logic
    }
}

This is probably for better and easier implementation, like you've implemented, so you don't have to worry about such stuff. If it was meant to throw error if timer is null/undefined because it would have some terrible side-effect, they'd probably implement it as

const clearTimeout = (timer) => {
    if (!timer) {
        throw new Error("Timer can not be undefined");
    }
}