how to set textbox value to empty(blank) using javascript

assuming the element you want to change has the id 'question' you can do

document.getElementById("question").value = "";

another example if 

var doc_val_check = $('#doc_title').attr("value");
    if (doc_val_check.length > 0) {
        doc_val_check == "";
    }

doc_val_check == "";   // == is equality check operator

should be
doc_val_check = ""; // = is assign operator. you need to set empty value // so you need = You can write you full code like this:
var doc_val_check = $.trim( $('#doc_title').val() ); // take value of text // field using .val() if (doc_val_check.length) { doc_val_check = ""; // this will not update your text field } To update you text field with a "" you need to try
$('#doc_title').attr('value', doc_val_check); // or $('doc_title').val(doc_val_check); But I think you don't need above process.

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In short, just one line

$('#doc_title').val("");

Note

.val() use to set/ get value in text field. With parameter it acts as setter and without parameter acts as getter.