How to replace space to underscore at same time (when i enter the text in input field)?

Question

can you please tell me how to replace space to underscore at same time (when i enter the text in input field)? If I have string I used like that.

replace(/ /g,"_");

But I am looking for when user enter text on input field then It automatically replace the space with underscore .I used keyup event it only fire first time.

$("#test").keyup(function() {
  var textValue = $(this).val();
    if(textValue==' '){
        alert("hh");
    }
});

Answer 1

DEMO

$("#test").keyup(function () {
    var textValue = $(this).val();
    textValue =textValue.replace(/ /g,"_");
    $(this).val(textValue);
});

Update

DEMO

$("#test").keyup(function () {
    this.value = this.value.replace(/ /g, "_");
});

Answer 2

Simply try this Demo

$("#test").keyup(function() {
    if ($(this).val().match(/[ ]/g, "") != null) {
        $(this).val($(this).val().replace(/[ ]/g, "_"));
    }
});