How can I run a javascript function when input changes

When a user creates a new account on my website I want to check if the name already exists. I want this check to run when they leave the username input box and go to enter a password.

I tried :

<开发者_如何学运维input type="text" onkeyup="check_user(this.value)"/>

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Call the function on the change event of text field:

<input name="username" onChange="check_user(this.value);" type="text"/>

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You need to use the "onblur" event. It fires when a control loses its focus, which seems to be exactly what you need.

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Because you used onkeyup in your code example, it sounds like you are looking to call a function on every keystroke. If that is the case, then instead of onchange - which fires when the input loses focus - you should use oninput which will fire anytime there is a change inside of input.

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I think you are looking for check the duplication of user. you have to write server side code for that.

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If you want to fire when something specific is type into the window:

phr = 'lol'
ltr = phr.split('')
text = ''

document.addEventListener('keyup',function(k){
    if (ltr.includes(k.key)) {
        text += k.key
        if (text == phr) {
            console.log('Typed!')
            text = ''
        }
    } else {
        text = ''
    }})

As a function your can use it like:

function wtt(phrase,clb) {
    phr = phrase
    ltr = phr.split('')
    text = ''

    document.addEventListener('keyup',function(k){
        if (ltr.includes(k.key)) {
            text += k.key
            if (text == phr) {
                clb()
                text = ''
            }
        } else {
            text = ''
        }})
}

wtt('my phrase', function () {
    console.log('dostuff')
})