Why does setting an input value to undefined not empty it?

Question

As demonstrated in the following example:

document.getElementById('test1').value = null;
document.getElementById('test2').value = undefined;

<input id="test1" value="DEFAULT TEXT" />
<input id="test2" value="DEFAULT TEXT" />

An input's value is reset whenever it is set to null, but why doesn't it also reset when it is set to undefined?

How come it sets the value to the string "undefined" instead?

That's how it is [defined](https://www.w3.org/TR/html50/forms.html#htmlinputelement) for the `value` property of the `HTMLInputElement`: `[TreatNullAs=EmptyString] attribute DOMString value;` — Andreas, Jan 25 '19 at 19:19
@andreas thats the first real answer to the question (unfortuantely posted as a comment) :) — Jonas Wilms, Jan 25 '19 at 19:21
For those who are voting to close as "too broad": the answers seem pretty concise and on-point. — Federico klez Culloca, Jan 25 '19 at 19:21
@Andreas Now that is the clarity I needed, this should definitely be an answer. — Mystical, Jan 25 '19 at 19:24
@federico not even that, the question is not broad in any way, there is probably a good dupe somewhere on SO that no one is able to find, but this is definetly not to broad... — Jonas Wilms, Jan 25 '19 at 19:27
@EternalDarkness It's now in Jonas answer. No need for another one^^ — Andreas, Jan 25 '19 at 19:28

Jonas Wilms · Accepted Answer · 2019-01-25T19:25:23.887

6

.value is a string and expects a string, so the correct way to empty it would be to use an empty string:

 /*...*/.value = "";

Assigning undefined will cause it to be stringified, which results in "undefined", whereas null cannot be stringified, therefore the spec says, that assigning null to .value will be like using an empty string.

edited Jan 25 '19 at 19:25

answered Jan 25 '19 at 19:20

Jonas Wilms

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score 2 · Answer 2 · edited Jan 25 '19 at 19:18

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undefined means a variable has been declared but has not yet been assigned a value.

On the other hand, null is an assignment value. It can be assigned to a variable as a representation of no value.

edited Jan 25 '19 at 19:18

Federico klez Culloca

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answered Jan 25 '19 at 19:14

Web Nexus

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`undefined` can also be assigned to a value, as `undefined` is a global variable being underined. – Jonas Wilms Jan 25 '19 at 19:22
1

Properties and variables can be assigned the value of undefined. It’s what the OP is specifically asking about. But in most situations setting the “value” of an INPUT DOM element to undefined will result in it being cast to a String. The only difference here is the outcome of the cast between null and undefined. I think this answer adds more confusion than it helps. – Cooper Buckingham Jan 27 '19 at 18:32

Why does setting an input value to undefined not empty it?

2 Answers2