I am learning JS now and can not understand why 'use strict' directive does not work for a callback for setTimeout? As I know for use strict it should be undefined but always have global object.
function f() {
'use strict'
console.log(this === window)
}
function g() {
console.log(this === window)
}
setTimeout(g,1000) // expect true, have true
setTimeout(f,1000) // expect false, have trueWhen setTimout() invokes a function in the browser, it sets the this to window. That is why it is not affected by strict mode - the this value is provided, instead of being left unset.
The specification for setTimeout can be found in the HTML standard
The timer initialization steps, given a
WindowOrWorkerGlobalScopeglobal, a string orFunctionhandler, a number timeout, a list arguments, a boolean repeat, and optionally (and only if repeat is true) a number previousId, are:1. Let thisArg be global if that is a
WorkerGlobalScopeobject; otherwise let thisArg be theWindowProxythat corresponds to global.[...]
9. Let task be a task that runs the following substeps:
If id does not exist in global's map of active timers, then abort these steps.
If handler is a
Function, then invoke handler given arguments with the callback this value set to thisArg. If this throws an exception, catch it, and report the exception.[...]
In essence, the result is similar to calling the function like so:
function f(){
'use strict'
console.log(this)
}
f.call(window)You may think about this===Window when you call a function like f() in non-strict mode as a default value for this (passed value is undefined).
But there is also a way to provide this value explicitly e.g.
f.call(Window)
In that case this is set explicitly and no default value for non-strict this is needed. Both strict and non-strict functions will show the same Window.
Frankly speaking when you pass a function as a callback you do not know how that callback will be called and you need to check the code of a wrapper function.
setTimeout does something like that pseudofunction:
function setTimeout(callback, delay, param3, param4,...){
// waiting a timer
callback.call(Window, param3, param4, ...)
}
P.S. If you are interested in I posted couple of years ago about that - https://dev.to/smlka/easyspec-how-does-settimeout-invoke-a-callback-function-in-a-browser-44kh