<pre class="shiki github-dark shaku"><code>function log(a,b,c,d) { console.log(a,b,c,d) // 1,2,3,undefined arguments[0] = 'bfe' arguments[3] = 'dev' console.log(a,b,c,d) // "bfe",2,3,undefined}log(1,2,3)</code></pre>
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments">arguments</a> is an Array-like object available inside functions containing the values of the arguments passed to it.
Each argument can also be set or reassigned provided arguments are passed while invoking.
Think of this as a static array created at the time of execution of the function. You can only set/update a value that is passed; changing something not passed or out of bounds makes no difference.
In this example, the function log expects 4 arguments but we are passing only 3 arguments. Hence, arguments inside this function is an array where only 3 elements are present and the last one is assumed to be <code>undefined</code> and you cannot update it as it was not explicitly passed while invoking the function.

arguments explained

What does the code snippet to the right output by console.log?…