KnockoutJS中的验证代码不起作用

Question

This is an example of validation for two input fields, firstname and lastname that both the fields are required

View

<p data-bind="css: { error: firstName.hasError }">
    FirstName :<input data-bind='value: firstName, valueUpdate: "afterkeydown"' required />
    <span data-bind='visible: firstName.hasError, text: firstName.validationMessage' > </span>
</p>
<p data-bind="css: { error: lastName.hasError }">
    LastName : <input data-bind='value: lastName, valueUpdate: "afterkeydown"' required/>
    <span data-bind='visible: lastName.hasError, text: lastName.validationMessage'> </span>
</p>

ViewModel

ko.extenders.required = function(target, overrideMessage) {
    //add some sub-observables to our observable
    target.hasError = ko.observable();
    target.validationMessage = ko.observable();

    //define a function to do validation
    function validate(newValue) {
       target.hasError(newValue ? false : true);
       target.validationMessage(newValue ? "" : overrideMessage || "This field is required");
    }

    //initial validation
    validate(target());

    //validate whenever the value changes
    target.subscribe(validate);

    //return the original observable
    return target;
};


function AppViewModel(first, last) {
    this.firstName = ko.observable(first).extend({ required: "Please enter a first name" });
    this.lastName = ko.observable(last).extend({ required: "" });
}

ko.applyBindings(new AppViewModel("Bob","Smith"));

Css

.error
{
  background-color: #f5f5f5; 
}

I tried running this code in JSFiddle . But its not working, even the initial values are not shown in the input fields, FirstName and LastName .I'm not able to figure out the error.

Answer 1

Were you including the knockout library? it seems to work for me. https://jsfiddle.net/co2y5x60/

https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.0/knockout-min.js