The `arguments` object changes if parameters change

Question

I just discovered that the arguments object actually changes if one of the parameters change.

For example:

function some(a, b, c ){
  console.log(arguments);


  args = [ a, b, c ];
  a = new Date();

  console.log(arguments);
  console.log(args);
}

some(1,2,3 );

You will see that while args stays the same (expected behaviour), arguments actually change.

Questions:

Is this something that is well documented? If so, where?
Is there anything else I need to be careful about the arguments object?

possible duplicate of Why are JavaScript Arguments objects mutated by assignment to parameter? — Qantas 94 Heavy
– Qantas 94 Heavy, Commented Feb 1, 2014 at 3:13

elclanrs · Accepted Answer · 2014-02-01 02:56:14Z

4

This is specified in the ECMA standard sec-10.6:

For non-strict mode functions [...] the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

answered Feb 1, 2014 at 2:56

elclanrs

94.2k21 gold badges137 silver badges171 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

SimplGy Over a year ago

Wow. What an odd behavior. Thanks for the reference.

Collectives™ on Stack Overflow

The `arguments` object changes if parameters change

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related