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JavaScript Validation API

Constraint Validation DOM Methods

Property Description
checkValidity() Returns true if an input element contains valid data.
setCustomValidity() Sets the validationMessage property of an input element.

If an input field contains invalid data, display a message:

The checkValidity() Method

<input id="id1" type="number" min="100" max="300" required>
<button onclick="myFunction()">OK</button>

<p id="demo"></p>

<script>
function myFunction() {
  const inpObj = document.getElementById("id1");
  if (!inpObj.checkValidity()) {
    document.getElementById("demo").innerHTML = inpObj.validationMessage;
  }
}
</script>

Constraint Validation DOM Properties

Property Description
validity Contains boolean properties related to the validity of an input element.
validationMessage Contains the message a browser will display when the validity is false.
willValidate Indicates if an input element will be validated.

Validity Properties

The validity property of an input element contains a number of properties related to the validity of data:

Property Description
customError Set to true, if a custom validity message is set.
patternMismatch Set to true, if an element's value does not match its pattern attribute.
rangeOverflow Set to true, if an element's value is greater than its max attribute.
rangeUnderflow Set to true, if an element's value is less than its min attribute.
stepMismatch Set to true, if an element's value is invalid per its step attribute.
tooLong Set to true, if an element's value exceeds its maxLength attribute.
typeMismatch Set to true, if an element's value is invalid per its type attribute.
valueMissing Set to true, if an element (with a required attribute) has no value.
valid Set to true, if an element's value is valid.

Examples

If the number in an input field is greater than 100 (the input's max attribute), display a message:

The rangeOverflow Property

<input id="id1" type="number" max="100">
<button onclick="myFunction()">OK</button>

<p id="demo"></p>

<script>
function myFunction() {
  let text = "Value OK";
  if (document.getElementById("id1").validity.rangeOverflow) {
    text = "Value too large";
  }
}
</script>

If the number in an input field is less than 100 (the input's min attribute), display a message:

The rangeUnderflow Property

<input id="id1" type="number" min="100">
<button onclick="myFunction()">OK</button>

<p id="demo"></p>

<script>
function myFunction() {
  let text = "Value OK";
  if (document.getElementById("id1").validity.rangeUnderflow) {
    text = "Value too small";
  }
}
</script>

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