Simple understanding of arguments in JS

stay js Each function in has a built-in object ,arguments, It can replace any formal parameter , For example, without knowing the number of parameters .

arguments Is a special object , His attribute names are in the order of the parameters passed in , have access to arguments[i],i=0,1,2,3... And he has length attribute , Store the number of currently passed in parameters , You can think of it as a class array object , But not an array . As can be seen from the following procedure .

Three formal parameters were passed in , Namely 1,2,3, Separated by commas , Then we printed it arguments The type of object, And then use for Loop traverses three formal parameters .

And pay attention to fn(1,2,3) and fn([1,2,3]) The difference between

 function fn() {
            console.log(arguments);
            console.log(typeof arguments);
            for (var i = 0; i < arguments.length; i++) {
                console.log(arguments[i]);
            }
}
        fn(1, 2, 3);

Each comma is used to represent a formal parameter ,fn([1,2,3]) Indicates that what is passed in is a formal parameter , therefore arguments.length=1, Only arguments[0]=[1,2,3] Indicates that this array is a formal parameter , therefore arguments[1] for undefined.

  function fn() {
            console.log(arguments);
            console.log(typeof arguments);
            for (var i = 0; i < arguments.length; i++) {
                console.log(arguments[i]);
            }

            console.log(arguments[1]);
            
            console.log(arguments.length);
            
        }
        // fn(1, 2, 3);
        var a = [4, 5, 6];
        console.log(a.length);
        fn(a);