I use setAttribute():

<input type="text" id="example"> // Setup text field 
<script type="text/javascript"> 
  document.getElementById("example").setAttribute('value','My default value');
</script>

if your form contains an input field like

<input type='text' id='id1' />

then you can write the code in javascript as given below to set its value as

document.getElementById('id1').value='text to be displayed' ; 

Instead of using document.getElementById() you can use document.querySelector() for different cases

more info from another Stack Overflow answer:

querySelector lets you find elements with rules that can't be expressed with getElementById and getElementsByClassName

EXAMPLE:

document.querySelector('input[name="myInput"]').value = 'Whatever you want!';

or

let myInput = document.querySelector('input[name="myInput"]');
myInput.value = 'Whatever you want!';

Test:

document.querySelector('input[name="myInput"]').value = 'Whatever you want!';

<input type="text" name="myInput" id="myInput" placeholder="Your text">

If you are using multiple forms, you can use:

<form name='myForm'>
    <input type='text' name='name' value=''>
</form>

<script type="text/javascript"> 
    document.forms['myForm']['name'].value = "New value";
</script>

Try out these.

document.getElementById("current").value = 12

// or

var current = document.getElementById("current");
current.value = 12

The answer is really simple

// Your HTML text field

<input type="text" name="name" id="txt">

//Your javascript

<script type="text/javascript"> 
document.getElementById("txt").value = "My default value";
</script>

Or if you want to avoid JavaScript entirely: You can define it just using HTML

<input type="text" name="name" id="txt" value="My default value">

<input id="a_name" type="text" />

Here is the solution using jQuery:

$(document).ready(function(){
  $('#a_name').val('something');
});

Or, using JavaScript:

document.getElementById("a_name").value = "Something";

The simple answer is not in Javascript the simplest way to get the placeholder is through the place holder attribute

<input type="text" name="text_box_1" placeholder="My Default Value" />

As of today (2025-02-03), none of the other answers comes even close to adequately answer the question. And yes, this is much more complicated than the average developer would like it to be, and also than the question would make you expect.

Let's dive in and shed some light on the matter!

Setting the Default Value of an Input Element: value vs defaultValue, attribute vs property

It's important to understand the distinction between the value attribute and the value property (also sometimes called IDL attribute) when working with input elements in HTML and JavaScript. These two serve different purposes and understanding their roles will allow you to effectively manage the initial and current values of an input field.

1. The value Attribute

The value attribute is primarily used to set the initial value of an input element when the HTML is first parsed by the browser. This initial value is also known as the default value.

The corresponding JavaScript property for this attribute is not value, but rather defaultValue. Essentially, the defaultValue property is a reflection of the value attribute:

get defaultValue() { return this.getAttribute('value') ?? ''; }
set defaultValue(val) { this.setAttribute('value', val); }

Here, this refers to the input element. As you can see, the value attribute and the defaultValue property are always in sync.

However, there's a caveat: if the user has not yet interacted with the input field, programmatically setting the value attribute via setAttribute will also update the current value of the input, which is represented by the value property. This synchronization stops as soon as the user modifies the input field.

2. The value Property

The value property represents the current value of the input field.

If the input field had an initial value (set declaratively via the value attribute, in the HTML) and the user modifies this value, the value attribute and the value property will no longer be in sync. Once the input field is "dirty" (i.e., user-modified), setting the value attribute will no longer affect the current value.

3. value attribute vs defaultValue property vs value property - in real code

This code example demonstrates the behavior of the value attribute, defaultValue property, and value property. Experiment with the different buttons, observe the log output, and note how the behavior changes once you manually modify the input field's value.

const input = document.getElementById('input');

function inspectValues() {
  console.log('value attribute: ' + input.getAttribute('value'));
  console.log('defaultValue property: ' + input.defaultValue);
  console.log('value property: ' + input.value);
  console.log('--------------------------------------');
}

function setViaAttribute() {
  input.setAttribute('value', 'set via value attribute');
  inspectValues();
}

function setViaDefaultValue() {
  input.defaultValue = 'set via defaultValue property';
  inspectValues();
}

function setViaValueProperty() {
  input.value = 'set via value property';
  inspectValues();
}

<input id="input" value="foo">
<button type="button" onclick="inspectValues()">Inspect</button>
<button type="button" onclick="setViaAttribute()">Set via value attribute</button>
<button type="button" onclick="setViaDefaultValue()">Set via defaultValue property</button>
<button type="button" onclick="setViaValueProperty()">Set via value property</button>

document.getElementById("fieldId").value = "Value";

or

document.forms['formId']['fieldId'].value = "Value";

or

document.getElementById("fieldId").setAttribute('value','Value');

It's simple; An example is:

<input type="text" id="example"> // Setup text field 
<script type="text/javascript"> 
var elem = document.getElementById("example"); // Get text field
elem.value = "My default value"; // Change field
</script>

<form>
  <input type="number"  id="inputid"  value="2000" />
</form>


<script>
var form_value = document.getElementById("inputid").value;
</script>    

You can also change the default value to a new value

<script>
document.getElementById("inputid").value = 4000;     
</script>

If the field for whatever reason only has a name attribute and nothing else, you can try this:

document.getElementsByName("INPUTNAME")[0].value = "TEXT HERE";

This part you use in html

<input id="latitude" type="text" name="latitude"></p>

This is javaScript:

<script>
document.getElementById("latitude").value=25;
</script>

You can also try:

document.getElementById('theID').value = 'new value';

HTML:

<input id="smtpHost" type="user" name="smtpHost">

JS:

(document.getElementById("smtpHost") as HTMLInputElement).value = data.smtpHost;