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What is the arguments object in JavaScript?

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Arguments

arguments is an object that is accessible inside functions that contain the values of the arguments passed to that function.

function test(a, b, c) {
  console.log("Printing arguments: ", arguments[0], arguments[1], arguments[2]);
}
test(1,2,3);  //1, 2, 3

The arguments object is only available inside non-arrow functions. Although it isn’t an array, we can access elements using the index, and it has a length property that contains the number of arguments passed to the function.

function test(a, b, c) {
 console.log(arguments.length);
}
test(1); // 1
test(1, 2); // 2

The arguments objects contain all of the arguments passed during the function call, even if there are not as many parameters in the function declaration.

function test() {
 console.log(arguments.length);
}

test();  //0
test(1);  //1
test(1,2,3,5);  //4

We can update the value of arguments:

function test(a) {
  arguments[0] = 100;
  console.log(arguments[0]);
}

test(); //100
test(1); //100

If we change the value of the parameter inside a function without rest, default, or restructured parameters, it will be updated in the arguments object and vice versa.

function test(a, b) {
  arguments[0] = 100;
  console.log("a=>", a, "arg[0]=>", arguments[0]);

  b = 0;
  console.log("b=>", b, "arg[1]=>", arguments[1]);
}

test(1, 2);
 // a=> 100 arg[0]=> 100
 // b=> 0 arg[1]=> 0

But if we use rest, default, or restructured parameters in a function, the synchronization between the arguments and the variables will not happen.

function test(a = 10, b= 20) {
  arguments[0] = 100;
  console.log("a=>", a, "arg[0]=>", arguments[0]);

  b = 0;
  console.log("b=>", b, "arg[1]=>", arguments[1]);
}

test(1, 2);
 // a=> 1 arg[0]=> 100
 // b=> 0 arg[1]=> 2

Properties

length → contains the number of arguments passed to the function

callee → contains the currently executing function reference

function test() {
   console.log(arguments.callee);
   console.log(arguments.length);
}

test(1,2,3);

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